Soon many CAD-systems included boundary structures. However, although the implementation followed the same basic ideas, the implementations were different and exchange of volume models between CAD-systems was almost impossible. The answer to this was first introduction of national standards and de facto industrial standards (such as VDA-FS, SET and IGES). However, by the end of the 1980es the STEP (ISO 10303) standard was introduced and a common data structure for exchange of CAD volume models was introduced.  

The boundary structure has two types of objects:

• Topological objects:  Vertex, edge, loop, face, shell and volume
• Geometric objects: Point, curve and surface

• Volumes are bounded by shells
• Shells are built by faces
• Faces are limited by loop
• Loops are built by edges
• Edges are limited by vertices
• Faces and surfaces are related
• Edges and curves are related
• Vertices and points are related

[1] Braid, I.C., Designing with volumes, Ph.D. Thesis, Cambridge University, England, (1974).