My teacher wrote down something to day which I wish I had written down myself. It was about the crystal structure of wurtzite, which is an ABABABAB stacking of hexagonal lattices. What he wrote was two different specifications of the unit cell of wurtzite depending on which groups of elements in the periodic table were used to build the crystal. I think the first was something like (1/3,2/3,..) and I don't remember the next one. The reason I ask now is that I'm building a simulation of wurtzite crystals in a programme and would like to know what he meant exactly.