Crystalline solids have regular, repeating arrangements of atoms or molecules. In order to classify these arrangements, chemists use a three-dimensional lattice to encompass the smallest repeat unit (known as the unit cell) of the crystal structure. There is really a lot of math involved, but we can explain it visually. A unit cell is really just a tiny box containing some atoms or molecules. We draw the edges of this small box so that if you were to line up lots of identical copies of this box in all directions, you would get the structure of the entire crystal lattice.