What is pennyweight having to do with an ounce?

Answer 1

British coinage traditionally had 240 pennies to a pound. ie One Pound = 240 pence (It has since been decimalised and there are now 100 pence in a pound) Similarly a pennyweight of a precious metal like silver was 1/240 th of a troy pound weight. A troy pound is somewhat lighter than the avoirdupois pound used for everyday purposes like weighing fruit and vegetables. Although there are 16 ounces in an avoirdupois pound, there are only 12 troy ounces in a troy pound (the word ounce means a twelfth). So there are 20 pennyweights in a troy ounce. The pennyweight is still used but is no longer an official unit of weight.