CAD Glossary
B-rep (Boundary Representation)
How CAD systems define a solid — by the faces, edges, and vertices that bound it, with exact surface geometry.
B-rep (boundary representation) is the method CAD systems use to describe a solid by its boundary: the faces that enclose the volume, the edges where those faces meet, and the vertices where edges meet. Each face carries an exact mathematical surface — a plane, a cylinder, a NURBS patch — so the shape is defined precisely rather than approximated.
What a B-rep carries
- Exact surface geometry rather than a faceted approximation
- Topology — which faces bound the volume, and which edges join which faces
- Enough information to compute mass properties, sections, and toolpaths
What a B-rep does not carry
A B-rep records the finished shape, not how it was built. It has no feature history, so a hole is a cylindrical face bounded by edges rather than a hole feature with an editable diameter.
This is why a STEP file imported into a CAD system arrives as a solid you can measure, section, and machine from, but not one you can edit parametrically. Changing a bore diameter means surface editing rather than typing a new number.
B-rep and mesh formats
A mesh format such as STL approximates curved surfaces with flat triangles. A B-rep stores the true surface. Converting B-rep to mesh discards precision permanently. Going the other way, from mesh back to solid, is a reconstruction rather than a format conversion.