条
tiáo
If your eye can follow it, use 条
The picture A single strand your finger can trace from one end to the other. Straight or winding, it doesn't matter. 条 is one line your eye follows end to end; 件 is one item you take out and handle on its own — which is why trousers are 条 and shirts are 件.
Where the picture shows up in 条
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a jammed line
这条路很堵。
zhè tiáo lù hěn dǔ
This road is really backed up.
Looking down from a footbridge at a road drawn out into one line packed solid with cars.
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the line your legs run down
我买了一条裤子。
wǒ mǎi le yì tiáo kùzi
I bought a pair of trousers.
A pair of trousers on a hanger, hanging long from waist to ankle.
Tops take 件; trousers and skirts take 条.
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a line on your screen
我给你发了一条消息。
wǒ gěi nǐ fā le yì tiáo xiāoxi
I sent you a message.
A message surfacing across the top of your phone screen as one strip.
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a line moving through water
他钓到了一条大鱼。
tā diào dào le yì tiáo dà yú
He caught a big fish.
A fish coming out of the water stretched long on the end of the line.
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a line running across the land
这条河的水很干净。
zhè tiáo hé de shuǐ hěn gānjìng
The water in this river is clean.
A river seen from a hilltop, winding away as a single line.
All of it in one picture
The jammed road is a line the eye follows, the trousers on the hanger are a line, the message on the screen is a line, the fish drawn out of the water is a line, and the winding river is a line. Road, clothing, words, living thing — 条 never asks what it is; it asks only whether your finger could trace it from one end to the other. So anything counted with 条 keeps the feeling of stretching from one end to the other. Straight or winding makes no difference.