Definition:Rubik's Cube/Center Facet/Linguistic Note

Linguistic Note on Center Facet of Rubik's Cube
In the notation $S_Z$ for the center facets of Rubik's cube, the subscript $Z$ is for zentrum, which is German for center.

$C$ cannot be used, because $S_C$ is already in use as the symbol denoting the set of corner facets.