hP

In the trigonal and hexagonal crystal systems, the rotation axis of order 3 or 6 (along the c-direction) constrains the unit-cell angles a = b = 90° and g = 120° and the unit-cell lengths so that a = b, as shown. Note that the hexagonal Bravais lattice is associated with two crystal systems in contrast to the other Bravais lattices. The unit cell axes of this Bravais lattice are often referred to as hexagonal cell axes.