Il y a deux manières de poser un plan compact sur un autre. Si l'on empile trois plans compacts, on a donc deux solutions :

• aucun plan ne se superpose (ABC), l'empilement est dit « cubique à faces centrées » (cfc) ;
• le troisième plan se superpose au premier (ABA), l'empilement est dit « hexagonal compact » (hc).

There are two ways to put a compact plane atop of another. When three compact planes are piled up, there are thus two situations:

• the planes cannot be superimposed (ABC), this is a "face centered cubic" (fcc) structure;
• the third plane is superimposed on the first one (ABA), this is an "hexagonal close-packing" (hcp).

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