Since the M-5 has been the class standard for decades why change it? The M-5M is still the M-5 main but now, instead the M-5 has been recreated from a "pattern" sail to a mould sail. This means the same designed, fast shape of the M-5 with smoother seams making a sail that performs as well (or better) than the M-5 and looks that much better doing it!