双
shuāng
Useless alone: 双
The picture A set that only works when the two sit side by side. Left with one, it stops being useful. 双 is the set that has to be two to be one; 只 is the single half taken out of it.
Where the picture shows up in 双
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one pair at the shop
我要买一双运动鞋。
wǒ yào mǎi yì shuāng yùndòngxié
I'd like to buy a pair of trainers.
Setting two trainers side by side into the box at the shop.
To point at just one of them, use 只.
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one set on the table
请再给我一双筷子。
qǐng zài gěi wǒ yì shuāng kuàizi
Could I get one more pair of chopsticks?
Tapping the ends of two chopsticks on the table to line them up.
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one set for winter
冬天要戴一双厚手套。
dōngtiān yào dài yì shuāng hòu shǒutào
In winter you need a thick pair of gloves.
Pushing a glove onto each hand, one per side.
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the pair on a face
她有一双很大的眼睛。
tā yǒu yì shuāng hěn dà de yǎnjing
She has really big eyes.
Two eyes sitting level on either side of the nose.
All of it in one picture
The trainers only go in the box as two matched halves, the chopsticks only work once both are lined up, the gloves only do their job with one on each hand, and eyes are a pair only when both sit level. 双 does not count the number two; it counts what has to be two before it is one. So wherever 双 turns up, there is the sense that a single half would be useless. And when you do point at that single half, the word switches to 只.