 Face-centred cubic selection rule

For equivalent scatterers on the four conventional fcc sites, the centering factor is $1+e^{i\pi (h+k)}+e^{i\pi (h+l)}+e^{i\pi (k+l)}$. It is four when the integer indices are all even or all odd, and zero otherwise. An additional atomic basis of a crystal can impose further extinctions. This rule concerns conventional cubic indexing.