つ
tsu
Counting on your fingers: つ
The picture Holding up fingers one at a time, without naming what kind of thing it is. つ counts without giving the thing a name; 個 names it first — a solid lump in the hand — and then counts.
Where the picture shows up in つ
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one in your hand
コーヒーを一つください。
kōhī o hitotsu kudasai
One coffee, please.
A finger goes up as you point at the coffee.
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that makes two
お弁当をふたつ、温めてください。
o-bentō o futatsu, atatamete kudasai
Two bento boxes, heated up please.
Two bento boxes set side by side on the counter, two fingers held up.
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counting years
うちの子は今年五つです。
uchi no ko wa kotoshi itsutsu desu
My kid turns five this year.
A kid holding up all five fingers of one hand.
つ counts ages only up to ten; from eleven on you switch to 歳 (さい).
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one thing with no shape
確認したいことが一つあります。
kakunin shitai koto ga hitotsu arimasu
There's one thing I'd like to check.
One finger raised as you say: just the one thing.
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the third one along
三つ目の信号を左に曲がってください。
mittsu-me no shingō o hidari ni magatte kudasai
Turn left at the third light.
A finger ticking off traffic lights through the windscreen — one, two, three.
All of it in one picture
The coffee is one finger, the bentos make two, the child's age is a whole hand, the question is one, and the lights get ticked off to three. Cups, boxes, years, questions, traffic lights — つ never asks which kind of thing it is; it just puts up another finger. So anything counted with つ still feels unlabelled, held up before you decide what to call it. That is why つ is the safe escape when the proper counter won't come to you.