Jigger

A two-sided measuring cup used to portion cocktail ingredients.

The modern bar jigger holds 1 oz (30 ml) on one side and 2 oz (60 ml) on the other, with graduated internal marks at 15, 22, 30, 45, and 60 ml. A $15 jigger eliminates roughly half the possible failure modes of a cocktail — it is the highest-return purchase in home bartending.