Why is there one octahedral and two tetrahedral positions for every anion in a close packed structure?

I'm reading about ionic crystal structures with planes of close-packed anions and the interstitial sites for cations. There are two types of interstitial sites: tetrahedral and octahedral. That makes perfect sense. What confuses me is this:



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> for each of these anion spheres, one octahedral and two tetrahedral positions exist.
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Where does this come from? It would seem that for every new anion sphere added, there is a single new tetrahedral position and a single new octahedral position.


