The dihedral group D 4 of symmetries of a square is a finite group of order 8. In this group, the order of r 1 is 4, as is the order of the subgroup R that this element generates. The order of the reflection elements f v etc. is 2. Both orders divide 8, as predicted by Lagrange's theorem. The groups F p × × of multiplication modulo a prime p have order ⁠ p − − 1 ⁠.