In hexagonal close packing (HCP) the second layer appears over depressions of the first layer.In HCP the third layer lies in registry with the first layer, thus the ABABAB... pattern.In cubic close packing (CCP) the second layer appears over depressions of the first layer.In CCP the third layer appears in depressions of the second layer in such a way as NOT to appear directly over the first layer.In CCP, the fourth layer lies in registry with the first layer, thus the ABCABC... pattern.
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