 Dihedral group

The dihedral group $D_{2n}$ is generated by a rotation $r$ of order $n$ and a reflection $s$ with $srs=r^{-1}$; it has order $2n$.

Our notation: $D_n$, called "dihedral group of degree n", means the dihedral group of the regular polygon with $n$ sides, and therefore has order $2n$ (all rotations + flips), called the "dihedral group of order 2n".