-------------------------------- The Combinatorial Geometry used by FLUKA is based on two important concepts: bodies and regions. The first ones are closed solid bodies (spheres, parallelepipeds, etc.) or semi-infinite portions of space (half-spaces, infinite cylinders) delimited by surfaces of first or second degree. The user must combine bodies by boolean operations (addition, intersection and subtraction) to perform a complete partition of the space of interest into regions, namely cells of uniform material composition. One important rule to remember is that inside the space of interest, defined by means of an external closed body, every point must belong to one and only one region. Input for the geometry description, which has its own format and rules, explained in Chap. 9, must be contained between a GEOBEGIN and a GEOEND card. These two cards follow the normal FLUKA input syntax. An option offered by the GEOBEGIN command is to read the geometry input from a separate file. Command GEOEND can be used also to invoke the geometry debugger, a check which is always strongly recommended. Geometry input, sandwiched between a GEOBEGIN and a GEOEND card can be placed anywhere in the input file (before the START command). It is mandatory in all cases. An optional command related to geometry is PLOTGEOM. It is used to display sections of the geometry and needs to read its own input, as explained later.