双 (shuāng) is a classifier for things that naturally come in twos, like shoes, socks, hands, or chopsticks. It differs from 对 (duì), which also means 'pair' but is used for matching items that are not necessarily attached or symmetrical, such as a pair of earrings or a couple. Use 双 when the two parts form a set, and 对 when they are two separate but matching objects.