What is the difference between avoirdupois measurements and troy measurements?

Troy weight is a system of mass units used primarily to measure gold and silver. A troy ounce is 480 grains, or 31.1 grams. Avoirdupois (pronounced AV-er-deh-POIZ) weight is a system of units that is used to measure mass, except for precious metals. It is based on the pound, which is approximately 454 grams. In both systems, the weight of a grain is the same: 65 milligrams.