\begin{definition}[Definition:Rubik's Cube/Center Facet]
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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:
:$S_Z = \set {F, U, R, L, B, D}$
\end{definition}