Jiggers equip bartenders to pour consistent shots, keeping liquor costs in control and servings consistent from drink to drink. The jigger is named for a portion of liquor that equals one and a half ounces, but bar jiggers come in many different shot measurement sizes. The common double jigger lets you measure two portion sizes. The most popular measures a half ounce in one cup and a full ounce in the other. Others measure three-quarters of an ounce in one cup and one and a quarter of an ounce in the other. Several other variations are available so you'll find one to prepare each of your famous cocktails.