对 duì - measure word

一对 [yī duì], measure word, means a couple of, a pair of

一对, used for a couple of

一对夫妇 [fū fù] a married couple 

一对夫妻 [fū qī] a married couple

一对青年男女 [qīnɡ nián nán nǚ] a young couple

一对双胞胎 [shuānɡ bāo tāi] a set of twins

一对, used for a pair of

一对蝴蝶 [hú dié] a pair of butterflies

一对鹦鹉 [yīnɡ wǔ] a pair of parrots

一对鸳鸯 [yuān yānɡ] a pair of Mandarin ducks

一对花瓶 [huā pínɡ] a pair of vases

一对耳环 [ěr huán] a pair of earrings

一对网球拍 [wǎnɡ qiú pāi] a pair of tennis rackets

一对椅子 [yǐ zi] a pair of chairs

一对姊妹城市 [zǐ mèi chénɡ shì] a pair of sister cities

Though 一对 [yī duì] and 一双 [yī shuānɡ] both mean “a pair of,” 一双 [yī shuāng] is largely used for a pair of body organs, limbs, or two things of the same kind often used together; whereas 一 对 [yī duì] is used for two people or two things that are seen together and associated with one another. 

One exception is 裤子 [kù zi, pants]. The measure word for 裤子 [kù zi] is 一条[yī tiáo]. For 手套 [shǒu tào, gloves] and 耳环 [ěr huán, earrings], 一副/付 [yī fù] is more often used than 一双 [yī shuānɡ].