the stacking sequence

The stacking sequence is the recipe that says, layer by layer, how close-packed planes are piled on top of one another. Give each possible sideways position of a close-packed layer a letter, A, B, or C, and the whole crystal becomes a string like ABCABCABC. That string is a compact way of naming a structure and of spotting when something has gone wrong.

Two sequences dominate. FCC is ...ABCABC..., a three-layer repeat where each layer shifts to a fresh position before returning. HCP is ...ABAB..., a two-layer repeat where every second layer sits over the first. A mistake in the recipe, say ...ABCABABC..., is a stacking fault: a thin slab of the wrong stacking wedged into an otherwise perfect crystal. The energy cost of such a slip is the stacking-fault energy, and it turns out to control how dislocations split and how easily a metal work-hardens.

Because only the stacking distinguishes FCC from HCP, and many long periodic sequences are possible (ABCACB, and much longer), the stacking sequence is the natural language for talking about polytypes such as silicon carbide, where dozens of structures differ only in how the identical layers are stacked. Read the sequence and you have named the structure.