A volume can be declared to have contents and as such it becomes a mother volume. The contents are either predefined volumes which are explicitly positioned inside the mother, or new volumes which are implicitly defined by a division mechanism applied to the mother. Positioning a volume with given shape and dimensions inside a mother volume is obtained by specifying its translation and rotation with respect to the mother reference frame. The user should make sure that no volume extends beyond the boundaries of its mother.
When a volume is positioned, the user gives it a number. Multiple copies of the same volume can be positioned inside the same or different mothers as long as their copy numbers differ. The contents of the positioned volume are reproduced in all copies.
Mother volumes can be divided by planes along the three axes ( x, y, z), radially along , or along the spherical coordinates of a polar system: R, and . The axis along which it is possible to perform a division depend on the shape to be divided. The general rule is that the result of the division be still a GEANT shape.
A volume can be partially or totally divided. The division process generates a number of cells, which are considered as new volumes. The cell dimensions are computed according to the number of divisions or the step size. A cell, as any other volume, can again be divided, or have other volumes positioned into it. Any operation of positioning or division performed on one cell is repeated to all the cells resulting from the division. A volume can be either divided or have contents, but not the two things together.
These operations define a physical tree with several levels. The material and properties of the contents replace the ones of the mother within the space region they occupy. A volume is therefore defined not only by its intrinsic characteristics but also by those of its descendants, namely its contents (by division or positioning), the contents of its contents, etc.