Hexagonal Close Packed, hcp
Hexagonal close packing is one of two crystal structures that derive from stacking close-packed-planes of hard sphere atoms. In the HCP structure only two stacking locations are employed in the sequence which can be represented as .....ABABABABAB....... packing. The lattice of the hexagonal close packed crystal is hexagonal and a basis of two atoms is placed in the same orientation on each lattice point. The diagrams below illustrate the geometry and space filling of this crystal structure.

The hcp structure has the same nearest neighbor configuration as the fcc close packed structure that uses the three possible stacking locations in an .....ABCABCABCABC..... sequence. The next-nearest-neighbors (nnn) differ in these two crystals. 

From: Newey and Weaver, "Materials Principles and Practice," Butterworth (1990)