In a crystal, atoms are arranged in straight rows in a three-dimensional periodic pattern. A small part of the crystal that can be repeated to form the entire crystal is called a unit cell.

Asymmetric unit

Primitive unit cell

Conventional unit cell

Crystal

A primitive unit cell is the smallest pattern that can be repeated to form the whole crystal. Crystalographers sometimes use a somewhat larger unit cell called the conventional unit cell which makes it easier to visualize the crystal. For crystals with cubic symmetry, the conventional unit cell is the smallest cube that can be repeated to form the whole crystal.

The asymmetric unit is the smallest pattern of atoms that can be used to construct the primitive unit cell by applying the symmetries of the crystal to the asymmetric unit.