Each normal mode of molecular vibration has a symmetry which forms a basis for one irreducible representation of the molecular symmetry group. For example, the water molecule has three normal modes of vibration: symmetric stretch in which the two O-H bond lengths vary in phase with each other, asymmetric stretch in which they vary out of phase, and bending in which the bond angle varies. The molecular symmetry of water is C 2v with four irreducible representations A 1, A 2, B 1 and B 2. The symmetric stretching and the bending modes have symmetry A 1, while the asymmetric mode has symmetry B 2. The overall symmetry of the three vibrational modes is therefore Γ vib = 2A 1 + B 2.