For the hexagonal lattice, there is a peculiar and redundant convention for specifying the Miller indices. First, one determines the intercepts of the plane in question along four axes, as defined by the vectors shown in the figure. These are placed in the order:
intercept along a1 axis, along a2 axis, along d axis, along a3 axis.
The rest of the procedure is the same as for other lattices, i.e. we take the inverse of each number, then find the smallest set of integers with the same ratio. This results in the "Miller-Bravais" index, which has the form (hkil), where the numbers h, k, and l are exactly the same as for the regular procedure. As shown below, i = -h -k.