diff --git a/src/ifcopenshell-python/ifcopenshell/api/alignment/__init__.py b/src/ifcopenshell-python/ifcopenshell/api/alignment/__init__.py
index 7914f982ba..56a158842c 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/alignment/__init__.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/alignment/__init__.py
@@ -91,6 +91,7 @@ from .layout_vertical_alignment_by_pi_method import (
 from .name_segments import name_segments
 from .update_end_point import update_end_point
 from .update_fallback_position import update_fallback_position
+from .update_key_point_referents import update_key_point_referents
 from .util import *
 
 __all__ = [
@@ -133,5 +134,6 @@ __all__ = [
     "register_referent_name_callback",
     "update_end_point",
     "update_fallback_position",
+    "update_key_point_referents",
     "get_mapped_segments",
 ]
diff --git a/src/ifcopenshell-python/ifcopenshell/api/alignment/_get_segment_start_point_label.py b/src/ifcopenshell-python/ifcopenshell/api/alignment/_get_segment_start_point_label.py
index db162d7d33..32dc5aa75d 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/alignment/_get_segment_start_point_label.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/alignment/_get_segment_start_point_label.py
@@ -26,9 +26,10 @@ _cant_callback = None
 
 def register_referent_name_callback(horizontal=None, vertical=None, cant=None):
     """
-    Referents are automatically created at the start of each horizontal, vertical, and cant segment.
-    The referents represent key points in the alignment layout such as Point of Curvature, Point of Tangent, and others.
-    Different juristicions use different naming systems for these key points.
+    Referents are created at the start of each horizontal, vertical, and cant segment by
+    ifcopenshell.api.alignment.update_key_point_referents. The referents represent key points in the
+    alignment layout such as Point of Curvature, Point of Tangent, and others. Different
+    juristicions use different naming systems for these key points.
 
     The referent name callback functions provide a customizable method for naming these referents. If a callback is registered,
     it is called when creating the referent name, otherwise the default naming is used.
@@ -39,8 +40,8 @@ def register_referent_name_callback(horizontal=None, vertical=None, cant=None):
 
     The callback function returns a string that is used in the referent name for the referent at the start of `segment`.
     The callback must accomodate the following cases:
-    * prev_segment = None and segment != None - this indicates the last segment so the "End of Alignment" name is returned
-    * prev_segment != None and segment == None - this indicates the first segment so the "Beginning of Alignment" name is returned
+    * prev_segment = None and segment != None - this indicates the first segment so the "Beginning of Alignment" name is returned
+    * prev_segment != None and segment == None - this indicates the last segment so the "End of Alignment" name is returned
     * prev_segment != None and segment != None - this indicates an intermediate segment so a name representitive of the transition is returned
 
     Setting any or all of the callbacks to None causes the default naming to be used.
diff --git a/src/ifcopenshell-python/ifcopenshell/api/alignment/_sort_nest.py b/src/ifcopenshell-python/ifcopenshell/api/alignment/_sort_nest.py
new file mode 100644
index 0000000000..eff06eebb2
--- /dev/null
+++ b/src/ifcopenshell-python/ifcopenshell/api/alignment/_sort_nest.py
@@ -0,0 +1,27 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2025 Thomas Krijnen <thomas@aecgeeks.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+from typing import Callable
+
+from ifcopenshell import entity_instance
+
+
+def _sort_nest(nest: entity_instance, key: Callable) -> entity_instance:
+    """Sorts the RelatedObjects of an IfcRelNests in place, by an arbitrary key function."""
+    nest.RelatedObjects = sorted(nest.RelatedObjects, key=key)
+    return nest
diff --git a/src/ifcopenshell-python/ifcopenshell/api/alignment/add_stationing_referent.py b/src/ifcopenshell-python/ifcopenshell/api/alignment/add_stationing_referent.py
index b3bc758dd3..19f13ca904 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/alignment/add_stationing_referent.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/alignment/add_stationing_referent.py
@@ -20,6 +20,7 @@ from typing import Optional
 
 import ifcopenshell
 import ifcopenshell.api.alignment
+from ifcopenshell.api.alignment._sort_nest import _sort_nest
 from ifcopenshell.api.alignment.update_fallback_position import update_fallback_position
 import ifcopenshell.api.pset
 import ifcopenshell.guid
@@ -122,8 +123,6 @@ def add_stationing_referent(
     else:
         nest.RelatedObjects += (referent,)
 
-    nest.RelatedObjects = sorted(
-        nest.RelatedObjects, key=lambda x: ifcopenshell.util.element.get_pset(x, name="Pset_Stationing", prop="Station")
-    )
+    _sort_nest(nest, key=lambda x: ifcopenshell.util.element.get_pset(x, name="Pset_Stationing", prop="Station"))
 
     return referent
diff --git a/src/ifcopenshell-python/ifcopenshell/api/alignment/update_key_point_referents.py b/src/ifcopenshell-python/ifcopenshell/api/alignment/update_key_point_referents.py
new file mode 100644
index 0000000000..b339942bda
--- /dev/null
+++ b/src/ifcopenshell-python/ifcopenshell/api/alignment/update_key_point_referents.py
@@ -0,0 +1,222 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2025 Thomas Krijnen <thomas@aecgeeks.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+from typing import Optional
+
+import ifcopenshell
+import ifcopenshell.api.alignment
+import ifcopenshell.api.pset
+import ifcopenshell.guid
+import ifcopenshell.util.alignment
+import ifcopenshell.util.element
+from ifcopenshell import entity_instance
+from ifcopenshell.api.alignment._get_segment_start_point_label import (
+    _get_segment_start_point_label,
+)
+from ifcopenshell.api.alignment._sort_nest import _sort_nest
+from ifcopenshell.api.alignment.update_fallback_position import update_fallback_position
+
+
+def _get_key_point_referent_nest(layout: entity_instance) -> Optional[entity_instance]:
+    """
+    Searches layout.IsNestedBy for the IfcRelNests whose RelatedObjects are IfcReferent.
+
+    This is distinct from both get_stationing_nest (scoped to the parent IfcAlignment, and
+    specifically the STATION/station-equation nest) and get_alignment_segment_nest (the *segment*
+    nest that also lives on layout.IsNestedBy, holding IfcAlignmentSegment, never IfcReferent).
+    """
+    for nest in layout.IsNestedBy:
+        for related_object in nest.RelatedObjects:
+            if related_object.is_a("IfcReferent"):
+                return nest
+    return None
+
+
+def _remove_referent(file: ifcopenshell.file, referent: entity_instance) -> None:
+    """Cleanly deletes a key-point IfcReferent: its Pset_Stationing, its ObjectPlacement (if
+    exclusively owned by it), and finally the referent itself."""
+    for inverse in list(file.get_inverse(referent)):
+        if inverse.is_a("IfcRelDefinesByProperties"):
+            ifcopenshell.api.pset.remove_pset(file, product=referent, pset=inverse.RelatingPropertyDefinition)
+
+    object_placement = referent.ObjectPlacement
+    if object_placement and file.get_total_inverses(object_placement) == 1:
+        referent.ObjectPlacement = None
+        ifcopenshell.util.element.remove_deep2(file, object_placement)
+
+    file.remove(referent)  # also strips referent out of any IfcRelNests.RelatedObjects referencing it
+
+
+def _create_key_point_referent(
+    file: ifcopenshell.file,
+    alignment: entity_instance,
+    curve: Optional[entity_instance],
+    label: str,
+    distance_along: float,
+    station: float,
+) -> entity_instance:
+    if curve and curve.is_a("IfcCompositeCurve") and 0 < len(curve.Segments):
+        object_placement = file.createIfcLinearPlacement(
+            RelativePlacement=file.createIfcAxis2PlacementLinear(
+                Location=file.createIfcPointByDistanceExpression(
+                    DistanceAlong=file.createIfcLengthMeasure(distance_along),
+                    OffsetLateral=None,
+                    OffsetVertical=None,
+                    OffsetLongitudinal=None,
+                    BasisCurve=curve,
+                )
+            ),
+        )
+        update_fallback_position(file, object_placement)
+    else:
+        object_placement = file.createIfcLocalPlacement(
+            PlacementRelTo=None,
+            RelativePlacement=file.createIfcAxis2Placement2D(
+                Location=file.createIfcCartesianPoint(alignment.ObjectPlacement.RelativePlacement.Location.Coordinates)
+            ),
+        )
+
+    name = f"{label} ({ifcopenshell.util.alignment.station_as_string(file, station)})"
+
+    referent = file.createIfcReferent(
+        GlobalId=ifcopenshell.guid.new(),
+        OwnerHistory=None,
+        Name=name,
+        Description=None,
+        ObjectType=None,
+        ObjectPlacement=object_placement,
+        Representation=None,
+        PredefinedType="POSITION",
+    )
+
+    pset_stationing = ifcopenshell.api.pset.add_pset(file, product=referent, name="Pset_Stationing")
+    ifcopenshell.api.pset.edit_pset(file, pset=pset_stationing, properties={"Station": station})
+
+    return referent
+
+
+def update_key_point_referents(
+    file: ifcopenshell.file,
+    layout: entity_instance,
+    rel_nests: Optional[entity_instance] = None,
+    clear: bool = False,
+) -> entity_instance:
+    """
+    Creates IfcReferent key-point markers for every segment transition in an alignment layout.
+
+    Labels are derived from _get_segment_start_point_label (e.g. "P.C.", "P.T.", "P.O.B.",
+    "P.V.C.", ...), with the station appended, e.g. "P.C. (145+98.32)". Different jurisdictions use
+    different naming systems for these key points -- register_referent_name_callback() lets a
+    caller override the default horizontal/vertical/cant labeling before calling this function; if
+    a callback is registered, its output is used here instead of the built-in labels. Referents are
+    nested to `rel_nests`, an IfcRelNests distinct from the layout's segment nest (found via
+    get_alignment_segment_nest) and from the alignment's stationing nest (found via
+    get_stationing_nest) -- key-point referents never belong in either of those.
+
+    :param layout: IfcAlignmentHorizontal, IfcAlignmentVertical, or IfcAlignmentCant
+    :param rel_nests: an existing IfcRelNests to (re)populate. May live anywhere (e.g. the parent
+        IfcAlignment, the layout, or elsewhere) -- the caller decides. If omitted, an existing
+        referent-nest already on `layout` is reused, or a new one is created and related to `layout`.
+    :param clear: if True, deletes all IfcReferent currently in rel_nests.RelatedObjects (and their
+        Pset_Stationing) before regenerating. If False (default), new referents are appended to
+        whatever already exists -- no deduplication.
+    :return: the IfcRelNests, with RelatedObjects sorted ascending by Pset_Stationing.Station
+
+    Example:
+
+    .. code:: python
+
+        horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+        nest = ifcopenshell.api.alignment.update_key_point_referents(model, horizontal)
+
+    Example, with custom labels for a jurisdiction that doesn't use the built-in abbreviations:
+
+    .. code:: python
+
+        def my_horizontal_labels(prev_segment, segment):
+            if prev_segment is None:
+                return "Start"
+            if segment is None:
+                return "End"
+            return "Curve Point"  # a name representative of the prev_segment -> segment transition
+
+        ifcopenshell.api.alignment.register_referent_name_callback(horizontal=my_horizontal_labels)
+        horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+        nest = ifcopenshell.api.alignment.update_key_point_referents(model, horizontal)
+        # nest.RelatedObjects[0].Name starts with "Start (" instead of the default "P.O.B. ("
+    """
+
+    expected_types = ["IfcAlignmentHorizontal", "IfcAlignmentVertical", "IfcAlignmentCant"]
+    if not layout.is_a() in expected_types:
+        raise TypeError(
+            f"Expected entity type to be one of {[_ for _ in expected_types]}, instead received {layout.is_a()}"
+        )
+
+    if rel_nests is None:
+        rel_nests = _get_key_point_referent_nest(layout)
+        if rel_nests is None:
+            rel_nests = file.createIfcRelNests(
+                GlobalId=ifcopenshell.guid.new(), RelatingObject=layout, RelatedObjects=()
+            )
+
+    if clear:
+        for referent in list(rel_nests.RelatedObjects):
+            _remove_referent(file, referent)
+        rel_nests.RelatedObjects = ()
+
+    segments = list(ifcopenshell.api.alignment.get_layout_segments(layout))
+    if segments and ifcopenshell.api.alignment.has_zero_length_segment(layout):
+        segments = segments[:-1]
+
+    if not segments:
+        _sort_nest(
+            rel_nests, key=lambda x: ifcopenshell.util.element.get_pset(x, name="Pset_Stationing", prop="Station")
+        )
+        return rel_nests
+
+    alignment = ifcopenshell.api.alignment.get_alignment(layout)
+    start_station = ifcopenshell.api.alignment.get_alignment_start_station(file, alignment)
+    curve = ifcopenshell.api.alignment.get_layout_curve(layout)
+    is_horizontal = layout.is_a("IfcAlignmentHorizontal")
+
+    new_referents = []
+    distance_along = 0.0
+    prev_segment = None
+    for segment in segments:
+        dp = segment.DesignParameters
+        seg_distance_along = distance_along if is_horizontal else dp.StartDistAlong
+
+        label = _get_segment_start_point_label(prev_segment, segment)
+        station = start_station + seg_distance_along
+        new_referents.append(_create_key_point_referent(file, alignment, curve, label, seg_distance_along, station))
+
+        if is_horizontal:
+            distance_along += dp.SegmentLength
+        else:
+            distance_along = dp.StartDistAlong + dp.HorizontalLength
+
+        prev_segment = segment
+
+    label = _get_segment_start_point_label(prev_segment, None)
+    station = start_station + distance_along
+    new_referents.append(_create_key_point_referent(file, alignment, curve, label, distance_along, station))
+
+    rel_nests.RelatedObjects = tuple(rel_nests.RelatedObjects) + tuple(new_referents)
+    _sort_nest(rel_nests, key=lambda x: ifcopenshell.util.element.get_pset(x, name="Pset_Stationing", prop="Station"))
+
+    return rel_nests
diff --git a/src/ifcopenshell-python/test/api/alignment/test_referent_names.py b/src/ifcopenshell-python/test/api/alignment/test_referent_names.py
index d64ea9a3ec..856e1ad197 100644
--- a/src/ifcopenshell-python/test/api/alignment/test_referent_names.py
+++ b/src/ifcopenshell-python/test/api/alignment/test_referent_names.py
@@ -97,16 +97,32 @@ def callback_alignment():
 
 
 def test_with_default_names(default_names_alignment):
-    referent_nest = ifcopenshell.api.alignment.get_referent_nest(None, default_names_alignment)
+    file = default_names_alignment.file
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(default_names_alignment)
+    vertical = ifcopenshell.api.alignment.get_vertical_layout(default_names_alignment)
 
-    expected = ["P.O.B", "P.C.", "P.T.", "P.O.E.", "V.P.O.B.", "P.V.C.", "P.V.T.", "V.P.O.E"]
-    for r in referent_nest.RelatedObjects:
-        assert [x in r.Name for x in expected]
+    h_nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    v_nest = ifcopenshell.api.alignment.update_key_point_referents(file, vertical)
+
+    expected_h = ["P.O.B.", "P.C.", "P.T.", "P.C.", "P.T.", "P.C.", "P.T.", "P.O.E."]
+    expected_v = ["V.P.O.B.", "P.V.C.", "P.V.T.", "P.V.C.", "P.V.T.", "P.V.C.", "P.V.T.", "P.V.C.", "P.V.T.", "V.P.O.E."]
+
+    assert [r.Name.split(" (")[0] for r in h_nest.RelatedObjects] == expected_h
+    assert [r.Name.split(" (")[0] for r in v_nest.RelatedObjects] == expected_v
 
 
 def test_with_callbacks(callback_alignment):
-    referent_nest = ifcopenshell.api.alignment.get_referent_nest(None, callback_alignment)
+    file = callback_alignment.file
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(callback_alignment)
+    vertical = ifcopenshell.api.alignment.get_vertical_layout(callback_alignment)
 
-    expected = ["A", "Q", "Z", "a", "q", "z"]
-    for r in referent_nest.RelatedObjects:
-        assert [x in r.Name for x in expected]
+    h_nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    v_nest = ifcopenshell.api.alignment.update_key_point_referents(file, vertical)
+
+    expected_h = ["A", "Q", "Q", "Q", "Q", "Q", "Q", "Z"]
+    expected_v = ["a", "q", "q", "q", "q", "q", "q", "q", "q", "z"]
+
+    assert [r.Name.split(" (")[0] for r in h_nest.RelatedObjects] == expected_h
+    assert [r.Name.split(" (")[0] for r in v_nest.RelatedObjects] == expected_v
+
+    ifcopenshell.api.alignment.register_referent_name_callback(None, None, None)  # reset global state
diff --git a/src/ifcopenshell-python/test/api/alignment/test_update_key_point_referents.py b/src/ifcopenshell-python/test/api/alignment/test_update_key_point_referents.py
new file mode 100644
index 0000000000..f58dc67443
--- /dev/null
+++ b/src/ifcopenshell-python/test/api/alignment/test_update_key_point_referents.py
@@ -0,0 +1,372 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2025 Thomas Krijnen <thomas@aecgeeks.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+from collections import Counter
+
+import pytest
+
+import ifcopenshell.api.alignment
+import ifcopenshell.api.context
+import ifcopenshell.api.unit
+import ifcopenshell.util.alignment
+import ifcopenshell.util.element
+
+COORDINATES = [(500.0, 2500.0), (3340.0, 660.0), (4340.0, 5000.0), (7600.0, 4560.0), (8480.0, 2010.0)]
+RADII = [1000.0, 1250.0, 950.0]
+VPOINTS = [(0.0, 100.0), (2000.0, 135.0), (5000.0, 105.0), (7400.0, 153.0), (9800.0, 105.0), (12800.0, 90.0)]
+LENGTHS = [1600.0, 1200.0, 2000.0, 800.0]
+
+
+def _new_file():
+    file = ifcopenshell.file(schema="IFC4X3")
+    file.createIfcProject(GlobalId=ifcopenshell.guid.new(), Name="Test")
+    length = ifcopenshell.api.unit.add_si_unit(file, unit_type="LENGTHUNIT")
+    ifcopenshell.api.unit.assign_unit(file, units=[length])
+    geometric_representation_context = ifcopenshell.api.context.add_context(file, context_type="Model")
+    ifcopenshell.api.context.add_context(
+        file,
+        context_type="Model",
+        context_identifier="Axis",
+        target_view="MODEL_VIEW",
+        parent=geometric_representation_context,
+    )
+    return file
+
+
+def _new_file_no_context():
+    file = ifcopenshell.file(schema="IFC4X3")
+    file.createIfcProject(GlobalId=ifcopenshell.guid.new(), Name="Test")
+    length = ifcopenshell.api.unit.add_si_unit(file, unit_type="LENGTHUNIT")
+    ifcopenshell.api.unit.assign_unit(file, units=[length])
+    return file
+
+
+def _build_alignment(file, start_station=0.0):
+    return ifcopenshell.api.alignment.create_by_pi_method(
+        file, "TestAlignment", COORDINATES, RADII, VPOINTS, LENGTHS, start_station
+    )
+
+
+def _pset_station(referent):
+    return ifcopenshell.util.element.get_pset(referent, name="Pset_Stationing", prop="Station")
+
+
+def test_wrong_layout_type_raises_type_error():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    with pytest.raises(TypeError):
+        ifcopenshell.api.alignment.update_key_point_referents(file, alignment)
+
+
+def test_default_rel_nests_created_when_none_provided():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+    segment_nest = ifcopenshell.api.alignment.get_alignment_segment_nest(horizontal)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+
+    assert nest.is_a("IfcRelNests")
+    assert nest.RelatingObject == horizontal
+    assert nest.id() != segment_nest.id()
+    assert len(nest.RelatedObjects) == 8
+    assert all(r.is_a("IfcReferent") for r in nest.RelatedObjects)
+
+
+def test_second_call_without_rel_nests_reuses_existing_nest():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+    segment_count_before = len(ifcopenshell.api.alignment.get_alignment_segment_nest(horizontal).RelatedObjects)
+
+    nest1 = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    nest2 = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+
+    assert nest1.id() == nest2.id()
+    assert len(nest2.RelatedObjects) == 16
+    segment_count_after = len(ifcopenshell.api.alignment.get_alignment_segment_nest(horizontal).RelatedObjects)
+    assert segment_count_after == segment_count_before
+
+
+def test_provided_rel_nests_is_used_as_is():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    # the nest may live anywhere the caller chooses, e.g. hung off the parent IfcAlignment
+    rel_nests = file.createIfcRelNests(GlobalId=ifcopenshell.guid.new(), RelatingObject=alignment, RelatedObjects=())
+
+    result = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal, rel_nests=rel_nests)
+
+    assert result.id() == rel_nests.id()
+    assert result.RelatingObject == alignment
+    assert len(result.RelatedObjects) == 8
+
+
+def test_clear_true_removes_old_referents_and_psets():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    old_referent_ids = [r.id() for r in nest.RelatedObjects]
+    old_pset_ids = [r.IsDefinedBy[0].RelatingPropertyDefinition.id() for r in nest.RelatedObjects]
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal, rel_nests=nest, clear=True)
+
+    assert len(nest.RelatedObjects) == 8
+    for old_id in old_referent_ids + old_pset_ids:
+        with pytest.raises(RuntimeError):
+            file.by_id(old_id)
+
+
+def test_clear_false_appends_without_dedup():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    ifcopenshell.api.alignment.update_key_point_referents(file, horizontal, rel_nests=nest, clear=False)
+
+    assert len(nest.RelatedObjects) == 16
+    counts = Counter(r.Name for r in nest.RelatedObjects)
+    assert len(counts) == 8
+    assert all(count == 2 for count in counts.values())
+
+
+def test_default_horizontal_labels_and_order():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+
+    expected = ["P.O.B.", "P.C.", "P.T.", "P.C.", "P.T.", "P.C.", "P.T.", "P.O.E."]
+    assert [r.Name.split(" (")[0] for r in nest.RelatedObjects] == expected
+
+    stations = [_pset_station(r) for r in nest.RelatedObjects]
+    assert stations == sorted(stations)
+    assert stations[0] == 0.0
+
+
+def test_default_vertical_labels_and_order():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    vertical = ifcopenshell.api.alignment.get_vertical_layout(alignment)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, vertical)
+
+    expected = [
+        "V.P.O.B.",
+        "P.V.C.",
+        "P.V.T.",
+        "P.V.C.",
+        "P.V.T.",
+        "P.V.C.",
+        "P.V.T.",
+        "P.V.C.",
+        "P.V.T.",
+        "V.P.O.E.",
+    ]
+    assert [r.Name.split(" (")[0] for r in nest.RelatedObjects] == expected
+
+    segments = ifcopenshell.api.alignment.get_layout_segments(vertical)
+    real_segments = segments[:-1] if ifcopenshell.api.alignment.has_zero_length_segment(vertical) else segments
+    # spot check the interior referents' stations against the segments' StartDistAlong directly
+    for referent, segment in zip(nest.RelatedObjects[1:-1], real_segments[1:]):
+        assert _pset_station(referent) == pytest.approx(segment.DesignParameters.StartDistAlong)
+
+
+def test_name_format():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    referent = nest.RelatedObjects[0]
+    station = _pset_station(referent)
+    assert referent.Name == f"P.O.B. ({ifcopenshell.util.alignment.station_as_string(file, station)})"
+
+
+def test_geometric_placement_when_layout_has_representation():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+    curve = ifcopenshell.api.alignment.get_layout_curve(horizontal)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+
+    for referent in nest.RelatedObjects:
+        assert referent.ObjectPlacement.is_a("IfcLinearPlacement")
+        location = referent.ObjectPlacement.RelativePlacement.Location
+        assert location.is_a("IfcPointByDistanceExpression")
+        assert location.BasisCurve == curve
+        assert referent.ObjectPlacement.CartesianPosition is not None
+
+    first, last = nest.RelatedObjects[0], nest.RelatedObjects[-1]
+    assert first.ObjectPlacement.RelativePlacement.Location.DistanceAlong.wrappedValue == pytest.approx(0.0)
+    assert last.ObjectPlacement.RelativePlacement.Location.DistanceAlong.wrappedValue == pytest.approx(
+        _pset_station(last)
+    )
+
+
+def test_fallback_placement_when_layout_has_no_geometry():
+    file = _new_file_no_context()
+    alignment = ifcopenshell.api.alignment.create(file, "A1", include_geometry=False)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+    ifcopenshell.api.alignment.layout_horizontal_alignment_by_pi_method(file, horizontal, COORDINATES, RADII)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+
+    expected_coordinates = alignment.ObjectPlacement.RelativePlacement.Location.Coordinates
+    for referent in nest.RelatedObjects:
+        assert referent.ObjectPlacement.is_a("IfcLocalPlacement")
+        assert referent.ObjectPlacement.RelativePlacement.Location.Coordinates == expected_coordinates
+
+
+def test_cant_layout_boundary_labels():
+    file = _new_file_no_context()
+    alignment = ifcopenshell.api.alignment.create(file, "A1", include_cant=True, include_geometry=False)
+    cant = ifcopenshell.api.alignment.get_cant_layout(alignment)
+
+    dp1 = file.createIfcAlignmentCantSegment(
+        StartDistAlong=0.0,
+        HorizontalLength=100.0,
+        StartCantLeft=0.0,
+        EndCantLeft=0.0,
+        StartCantRight=0.0,
+        EndCantRight=0.0,
+        PredefinedType="CONSTANTCANT",
+    )
+    ifcopenshell.api.alignment.create_layout_segment(file, cant, dp1)
+
+    dp2 = file.createIfcAlignmentCantSegment(
+        StartDistAlong=100.0,
+        HorizontalLength=50.0,
+        StartCantLeft=0.0,
+        EndCantLeft=0.0,
+        StartCantRight=0.0,
+        EndCantRight=0.0,
+        PredefinedType="CONSTANTCANT",
+    )
+    ifcopenshell.api.alignment.create_layout_segment(file, cant, dp2)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, cant)
+
+    labels = [r.Name.split(" (")[0] for r in nest.RelatedObjects]
+    assert labels[0] == "C.P.O.B."
+    assert labels[-1] == "C.P.O.E."
+    # CONSTANTCANT -> CONSTANTCANT is currently an unfilled "xx" placeholder in the cant lookup
+    # table (_get_segment_start_point_label.py) -- out of scope to fill in here.
+    assert labels[1] == "xx"
+
+    stations = [_pset_station(r) for r in nest.RelatedObjects]
+    assert stations == [0.0, 100.0, 150.0]
+
+
+def test_no_real_segments_produces_no_referents():
+    file = _new_file_no_context()
+    alignment = ifcopenshell.api.alignment.create(file, "A1", include_geometry=False)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    assert nest.RelatedObjects == ()
+
+
+def test_single_real_segment_produces_only_boundary_labels():
+    file = _new_file_no_context()
+    alignment = ifcopenshell.api.alignment.create(file, "A1", include_geometry=False)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    design_parameters = file.createIfcAlignmentHorizontalSegment(
+        StartTag=None,
+        EndTag=None,
+        StartPoint=file.createIfcCartesianPoint((0.0, 0.0)),
+        StartDirection=0.0,
+        StartRadiusOfCurvature=0.0,
+        EndRadiusOfCurvature=0.0,
+        SegmentLength=100.0,
+        GravityCenterLineHeight=None,
+        PredefinedType="LINE",
+    )
+    ifcopenshell.api.alignment.create_layout_segment(file, horizontal, design_parameters)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    labels = [r.Name.split(" (")[0] for r in nest.RelatedObjects]
+    assert labels == ["P.O.B.", "P.O.E."]
+
+
+def test_start_station_composes_for_child_alignment():
+    file = _new_file()
+    alignment = ifcopenshell.api.alignment.create(file, "A1", include_vertical=False, start_station=100.0)
+    ifcopenshell.api.alignment.add_vertical_layout(file, alignment)
+    ifcopenshell.api.alignment.add_vertical_layout(file, alignment)  # forces the child-alignment split
+
+    child_alignment = alignment.IsDecomposedBy[0].RelatedObjects[-1]
+    child_vertical = ifcopenshell.api.alignment.get_vertical_layout(child_alignment)
+
+    dp1 = file.createIfcAlignmentVerticalSegment(
+        StartDistAlong=0.0,
+        HorizontalLength=500.0,
+        StartHeight=10.0,
+        StartGradient=0.01,
+        EndGradient=0.01,
+        PredefinedType="CONSTANTGRADIENT",
+    )
+    ifcopenshell.api.alignment.create_layout_segment(file, child_vertical, dp1)
+
+    dp2 = file.createIfcAlignmentVerticalSegment(
+        StartDistAlong=500.0,
+        HorizontalLength=300.0,
+        StartHeight=15.0,
+        StartGradient=0.01,
+        EndGradient=0.01,
+        PredefinedType="CONSTANTGRADIENT",
+    )
+    ifcopenshell.api.alignment.create_layout_segment(file, child_vertical, dp2)
+
+    nest = ifcopenshell.api.alignment.update_key_point_referents(file, child_vertical)
+    stations = [_pset_station(r) for r in nest.RelatedObjects]
+    assert stations == pytest.approx([100.0, 600.0, 900.0])
+
+
+def test_returns_ifc_rel_nests():
+    file = _new_file()
+    alignment = _build_alignment(file)
+    horizontal = ifcopenshell.api.alignment.get_horizontal_layout(alignment)
+
+    result = ifcopenshell.api.alignment.update_key_point_referents(file, horizontal)
+    assert result.is_a("IfcRelNests")
+
+
+test_wrong_layout_type_raises_type_error()
+test_default_rel_nests_created_when_none_provided()
+test_second_call_without_rel_nests_reuses_existing_nest()
+test_provided_rel_nests_is_used_as_is()
+test_clear_true_removes_old_referents_and_psets()
+test_clear_false_appends_without_dedup()
+test_default_horizontal_labels_and_order()
+test_default_vertical_labels_and_order()
+test_name_format()
+test_geometric_placement_when_layout_has_representation()
+test_fallback_placement_when_layout_has_no_geometry()
+test_cant_layout_boundary_labels()
+test_no_real_segments_produces_no_referents()
+test_single_real_segment_produces_only_boundary_labels()
+test_start_station_composes_for_child_alignment()
+test_returns_ifc_rel_nests()