Immediately under this triangle, find a similar triangle of red spheres rotated 60 o with respect to the blue triangle. The space at the center of the 6 spheres is an octahedral hole . There are two ways to place a third layer of spheres above the first two. Using red spheres and the upper left blue layer, place the third layer over the first layer. Its more general designation is 1212. . . .
Hexagonal channels run vertically through the structure.
Using yellow spheres and the other blue layer, place the third layer so that neither red nor blue spheres lie below the yellow spheres. This gives a yellow triangle of 6 spheres, vertex down. Save these two models for later reference and proceed to the next part. Face-centered cubic lattice. Be sure you understand everything in a step before proceeding.