This implies that is the universal covering group of SO(4) — its unique double cover — and that and are normal subgroups of SO(4). The identity rotation and the central inversion form a group of order 2, which is the centre of SO(4) and of both and . The centre of a group is a normal subgroup of that group. The factor group of C2 in SO(4) is isomorphic to SO(3) × SO(3). The factor groups of 3L by C2 and of 3R by C2 are each isomorphic to SO(3). Similarly, the factor groups of SO(4) by 3L and of SO(4) by 3R are each isomorphic to SO(3).