In the frame of the GOCAD project, it has been decided to model surface (e.g. geophysical or geological interfaces) by connected triangles (J.L. Mallet, 1988, 1989). This very powerful approach allows for the modelling of any kind of 3D shapes, including complex shapes such as overhanging salt domes. It is also compulsory for a lot of applications to deal with volumes and not only with surfaces. Due to this fact, we have developed an extension to the Geometrical database of GOCAD, allowing the partition the 3D space into volumes or domains.