The most important aspect of any structure is its stoichiometry, the relative numbers of different types of atoms. The
stoichiometry of a unit cell can be determined by counting all the atoms depicted, and then taking account of those that
are shared with neighboring cells. Any atom at a corner of a unit cell is shared between eight cells, any at an edge
between four, and any on a face between two. Thus the composition MX in Fig. 1 is arrived at by counting the eight
corner M atoms, and then dividing by eight to account for sharing. With some experience, this procedure will seem
unnecessary. If one simply imagines the unit cell with a shifted origin as in Fig. 1c then it is immediately clear that every
cell contains one M and one X atom.
Another feature characteristic of a structure is the coordination of each atom. There is usually no difficulty in
seeing the coordination of an atom in the middle of a unit cell. (For example, X in Fig. 1a can easily be seen to have eight
M neighbors forming the corners of cube. In the projection, Fig. 1b. one needs to remember that the M atoms at the
base of the cell are repeated at the top.) For atoms at corners or edges it is necessary to consider what happens in
neighboring cells, and an extended drawing such as Fig. 1d may be helpful: this shows each M surrounded by eight X
neighbors in the same way as the coordination of X.