对
duì
Two that only make one when set facing: 对
The picture Two matching things set to face each other so that they lock into a single pair. 对 counts two that must face each other to make a pair; 双 counts two used together to do one job.
Where the picture shows up in 对
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two who live facing each other
他们是一对夫妻。
tāmen shì yí duì fūqī
Those two are husband and wife.
Two people seated across the table from each other at a meal.
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one on each side
我买了一对耳环。
wǒ mǎi le yí duì ěrhuán
I bought a pair of earrings.
Two identical earrings going in, one in each ear.
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two who arrived together
她生了一对双胞胎。
tā shēng le yí duì shuāngbāotāi
She had twins.
Two babies lying side by side under one blanket.
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two set symmetrically
门口放着一对花瓶。
ménkǒu fàng zhe yí duì huāpíng
There's a pair of vases by the door.
Identical vases placed one on each side of the doorway, facing in.
With only one in place, something looks missing.
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two standing side by side as one pair
这对新人很年轻。
zhè duì xīnrén hěn niánqīng
This newlywed couple is very young.
Two people standing shoulder to shoulder at the front of the hall.
All of it in one picture
A husband and wife living face to face are 一对, the earrings worn one in each ear are 一对, the twins born the same day are 一对. The vases set on either side of the door are 对, and so is the couple standing together at the wedding. 对 counts not the number two but 'a pair completed by being set opposite'. That is why, when one goes missing, the one left over is not simply one item but the half of a pair.