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:
Data structures describes how:
 Braid, I.C., Designing with volumes, Ph.D. Thesis, Cambridge University, England, (1974).
Published June 29, 2005