张
zhāng
If it lies flat, it's 张
The picture One thin surface spread out flat. A face you could sweep your palm across. 张 counts one face spread flat with its thickness ignored; 本 counts sheets bound into a block with a spine.
Where the picture shows up in 张
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one surface at the office
请给我一张纸。
qǐng gěi wǒ yì zhāng zhǐ
Could I have a sheet of paper?
Pulling one sheet from the stack by the printer and handing it over.
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two surfaces in your hand
我买了两张票。
wǒ mǎi le liǎng zhāng piào
I bought two tickets.
Two tickets fanned out between your fingers at the gate.
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a rectangular surface
这张照片是去年拍的。
zhè zhāng zhàopiàn shì qùnián pāi de
This photo was taken last year.
Holding a photo flat in both hands to look at it.
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the flat surface on legs
房间里有一张桌子。
fángjiān li yǒu yì zhāng zhuōzi
There's a desk in the room.
Sweeping your palm across the top of the desk in the room.
Desks and beds have a flat top, so they take 张, not 个.
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a card is a surface too
银行给了我一张新卡。
yínháng gěi le wǒ yì zhāng xīn kǎ
The bank gave me a new card.
Sliding a thin card out of the slot in your wallet.
All of it in one picture
The sheet you pull out is a surface, the two tickets fanned in your hand are surfaces, the photo you study is a surface, the desk on its legs is in the end one wide flat surface, and the card from your wallet is a surface too. 张 never asks what the thing is for; it asks only whether there is a broad face your palm could sweep across. So anything counted with 张 arrives with its thickness forgotten and its face spread open. That is why a desk taking 张 stops being strange.