Unit cells must have one of seven shapes:

Cubic: a = b = c; alpha = beta = gamma = 90 degrees

Monoclinic: a b c; alpha = gamma = 90 degrees beta

Hexagonal: a = b c; alpha = beta = 90 degrees; gamma = 120 degrees

Orthorhombic: ab c; alpha = beta = gamma = 90 degrees

Tetragonal: a = b c; alpha = beta = gamma =90 degrees

Rhombohedral: a = b = c; alpha = beta = gamma 90 degrees

Triclinic: abc; alphabeta gamma90 degrees

*where a, b, c are the lengths of the sides of the cell and alpha, beta, gamma are the angles between the sides