1.   The face-centred cubic lattice consists of a system of intersecting simple cubic unit cells.

2.   The body-centred cubic lattice consists of interpenetrating simple cubic unit cells.

3.   A typical crystal is the cubic lattice of table salt, in which atoms of sodium and chlorine alternate with each other in a kind of three-dimensional chess board.

4.   Hence, his claim that the face-centered cubic lattice is the most efficient arrangement became known as the Kepler conjecture.