The British traditional system of units of weight or mass (see Mass versus weight for the distinction) included several different types of pound, two of which were the Tower pound and the London pound. These pounds were divided into ounces, the same ounce being used for both, except that the London pound contained 16 ounces and the Tower pound 12. This common ounce was in turn divided into 20 pennyweights each of which contained 22½ troy grains.