The grain measurement, the earliest unit of weight measurement, used a grain of wheat or barley corn as its standard. It is the smallest unit of measure in the Troy system.

One grain also equals 64.798 of a milligram; 15.432 grains equals a gram.

It is also a unit used to measure pearls and equals 1/4 of a carat.

Grain also refers to cleavage direction in a diamond and patterns of color in an opal.

Troy Weight Abbreviation Comparisons
Grain gn 24 gn to 1 dwt
Pennyweight dwt 20 dwts to 1 Troy Ounce
Troy Ounce ozt 12 oz to 1 Troy Pound
Troy Pound lb. t. 1 lb. t. = 5,760 gn 1 lb. t. = 240 dwt 1 lb. t. = 12 ozt