Definition:Rubik's Cube/Center Facet

Definition

 * RubiksCubeFacets.png

Of each of the $9$ facets of each face of Rubik's cube, the ones in the centers of the faces are called the center facets.

The set of center facets can be denoted $S_Z$.

The set of center facets is:
 * $S_Z = \left\{ {F, U, R, L, U, D}\right\}$

Linguistic Note
The subscript $Z$ of $S_Z$ 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.