diff --git a/src/dofs/api.jl b/src/dofs/api.jl
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+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE
+
+"""
+DOF Type - Type-Level Field Specification
+
+`DOF{T, E}` is an **abstract type** used purely for type-level field specifications.
+
+# Philosophy
+
+DOF types are NEVER instantiated - they exist purely at the type level to specify
+field structure in a clean, readable way:
+
+```julia
+# Beautiful multi-field specification using DOFSet:
+@DOFSet{
+    u::DOF{Displacement{3}, Vertex},
+    p::DOF{Pressure, Cell}
+}
+```
+
+# Usage
+
+**Multi-field specification (preferred):**
+```julia
+S = @NamedTuple{
+    T::Tuple{Float64, Vertex},
+    u::Tuple{Vec{3,Float64}, Vertex},
+    p::Tuple{Float64, Cell}
+}
+
+# Or equivalently using DOF type syntax:
+(T = DOF{Float64, Vertex}, u = DOF{Vec{3,Float64}, Vertex}, p = DOF{Float64, Cell})
+```
+
+**Single-field specification:**
+```julia
+S = @NamedTuple{u::Tuple{Vec{3,Float64}, Vertex}}
+
+# Or using DOF type:
+(u = DOF{Vec{3,Float64}, Vertex},)
+```
+
+# Connection to Assembly
+
+The DOF specification `S` is a type parameter in Element:
+```julia
+Element{Tetrahedron{4}, Lagrange{1}, S}
+```
+
+The assembler uses `S` to:
+1. Compute total DOF count: `ndofs(S, Tetrahedron{4})`
+2. Extract field information: `fieldnames(S)`, `fieldtypes(S)`
+3. Count DOFs per field: `field_ndofs(field_type, topology)`
+4. Access element DOFs: `element.dof_indices.u`, `element.dof_indices.p`, etc.
+
+All type-level, compile-time resolved! No runtime overhead.
+"""
+
+# ============================================================================
+# DOF Size Function (utility for counting DOF components)
+# ============================================================================
+
+"""
+    dof_size(::Type{T}) → Int
+
+Number of DOF components for a quantity type.
+
+# Examples
+```julia
+dof_size(Float64)             # 1 (scalar)
+dof_size(Vec{2})              # 2 (2D vector)
+dof_size(Vec{3})              # 3 (3D vector)
+dof_size(Tensor{2,2})         # 4 (2×2 tensor)
+dof_size(Tensor{2,3})         # 9 (3×3 tensor)
+dof_size(SymmetricTensor{2,3}) # 6 (symmetric 3×3)
+```
+"""
+Base.@pure dof_size(::Type{Float64}) = 1
+Base.@pure dof_size(::Type{<:Vec{D}}) where {D} = D
+Base.@pure dof_size(::Type{<:Tensor{2,D}}) where {D} = D * D
+Base.@pure dof_size(::Type{<:SymmetricTensor{2,D}}) where {D} = div(D * (D + 1), 2)
+
+# ============================================================================
+# Abstract DOF Type (Base Type for All Field Specifications)
+# ============================================================================
+
+"""
+    abstract type AbstractDOF
+
+Abstract base type for all field specifications.
+
+# Purpose
+Base type for `DOF{T,E}` which is used inside `DOFSet` specifications:
+```julia
+S = @DOFSet{u::DOF{Displacement{3}, Vertex}}  # DOF{T,E} <: AbstractDOF
+Element{K, P, S}  # where S <: DOFSet
+```
+
+# Subtypes
+- `DOF{T,E}`: Type-level field specifications (never instantiated)
+
+# Interface
+Subtypes should support:
+- `Base.fieldnames(::Type{<:AbstractDOF})` - field names as tuple
+- `ndofs(::Type{<:AbstractDOF}, ::Type{<:AbstractTopology})` - total DOF count
+"""
+abstract type AbstractDOF end
+
+# ============================================================================
+# DOFSet - Multi-Field DOF Specification
+# ============================================================================
+
+"""
+    DOFSet
+
+Type alias for multi-field DOF specifications.
+
+**Current implementation**: NamedTuple (but this is an implementation detail!)
+
+Use `@DOFSet` macro to create multi-field specifications:
+```julia
+S = @DOFSet{T::DOF{Temperature, Vertex}, u::DOF{Displacement{3}, Vertex}}
+```
+
+# Purpose
+Hides implementation detail so we can change from NamedTuple to custom struct later
+without breaking user code.
+
+# Usage in Element Type
+```julia
+# Single-field - always use @DOFSet
+S = @DOFSet{u::DOF{Displacement{3}, Vertex}}
+Element{K, P, S}
+
+# Multi-field - use @DOFSet
+S = @DOFSet{T::DOF{Temperature, Vertex}, u::DOF{Displacement{3}, Vertex}}
+Element{K, P, S}
+```
+
+# Compatibility Note
+`NamedTuple` also works (since `DOFSet = NamedTuple`), but `DOFSet` is the preferred
+interface. Using `DOFSet` ensures your code will continue to work if we change the
+internal representation in the future.
+"""
+const DOFSet = NamedTuple
+
+"""
+    @DOFSet{field1::Type1, field2::Type2, ...}
+
+Create a multi-field DOF specification.
+
+# Syntax
+Each field is specified using `DOF{FieldType, EntityType}`:
+```julia
+field_name::DOF{FieldType, EntityType}
+```
+
+Where:
+- `FieldType`: AbstractField type (Displacement{3}, Temperature, etc.)
+- `EntityType`: Topological entity (Vertex, Edge, Face, Cell)
+
+# Examples
+```julia
+# Thermo-mechanical coupling
+S = @DOFSet{T::DOF{Temperature, Vertex}, u::DOF{Displacement{3}, Vertex}}
+
+# THM-E (full multi-physics)
+S = @DOFSet{
+    T::DOF{Temperature, Vertex},          # Temperature
+    u::DOF{Displacement{3}, Vertex},      # Displacement
+    p::DOF{Pressure, Cell},              # Pressure
+    φ::DOF{ElectricPotential, Edge}      # Electric potential
+}
+
+# Create element with multi-field spec
+element = Element{Tetrahedron{4}, Lagrange{1}, S}(id, dof_indices)
+```
+
+# Implementation Note
+Currently wraps `@NamedTuple` but this is hidden from users.
+Can be changed to custom struct later without breaking code!
+
+# Compatibility
+You can also use `@NamedTuple` directly, but `@DOFSet` is preferred for future compatibility.
+"""
+macro DOFSet(expr)
+    return :(@NamedTuple($expr))
+end
+
+# ============================================================================
+# DOF Type (Abstract - Type-Level Only)
+# ============================================================================
+
+"""
+    abstract type DOF{T, E<:TopologicalEntity} <: AbstractDOF
+
+Abstract type for type-level field specifications. NEVER instantiated!
+
+Used purely for clean syntax when specifying fields:
+```julia
+(u = DOF{Vec{3,Float64}, Vertex}, p = DOF{Float64, Cell})
+```
+
+This is equivalent to the NamedTuple type:
+```julia
+@NamedTuple{u::Tuple{Vec{3,Float64}, Vertex}, p::Tuple{Float64, Cell}}
+```
+
+# Type Parameters
+- `T`: Quantity type (Float64, Vec{D}, Tensor{2,D}, etc.)
+- `E`: Topological entity (Vertex, Edge, Face, Cell)
+
+# Examples
+```julia
+DOF{Float64, Vertex}              # Scalar at vertices
+DOF{Vec{3,Float64}, Vertex}       # 3D vector at vertices
+DOF{Tensor{2,3,Float64}, Cell}    # 3×3 tensor at cells
+DOF{SymmetricTensor{2,2}, Edge}   # 2×2 symmetric tensor on edges
+```
+"""
+abstract type DOF{T, E<:TopologicalEntity} <: AbstractDOF end
+
+# DOF size for DOF type (type-level query)
+Base.@pure function dof_size(::Type{<:DOF{T,E}}) where {T,E}
+    return dof_size(T)
+end
+
+# Fieldnames for single-field DOF types (default field name is :dof)
+Base.@pure Base.fieldnames(::Type{<:DOF}) = (:dof,)