· In assigning Miller indices to the hexagonal lattice a redundant coordinate system is used: 
(a₁, a₂, a₃, c). The basal plane is shown opposite.
· The same rules are used to define directions and planes in this system, but an additional (redundant) index is carried.
· A four index notation is used. The first three must sum to zero. In some notations, the redundant index, a₃ , is replaced by a period.