A ** unit cell** is a parallelepiped that is the simplest repeating unit of a crystal. A unit cell must fill all space of the crystal when replicated. As a result, spherical unit cells do not exist.

At each corner of the unit cell is a blue point known as a ** lattice point** (see above diagram). The environment of any lattice point is equivalent to the environment of any other lattice point in the crystal. An infinite three-dimensional array of lattice points forms a three-dimension

**called a**

*lattice***. A unit cell that only has lattice points at its corners is called a**

*space lattice***. Unit cells are sometimes chosen in the non-primitive form for convenience.**

*primitive unit cell*The edges of a unit cell are, by convention, chosen to be right-handed (** a** ×

**is the direction of**

*b***). The angles**

*c**α*,

*β*and

*γ*are between

*and*

**b***,*

**c***and*

**c***, and*

**a***and*

**a***respectively.*

**b**