One by one means one after another, separately. You use it about people or things that do something, or that something happens to, in turn rather than all at once: guests leave, lights go out in the windows, you work through your emails.
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verb + one by one; one by one, + plural subject + verb; compare: one at a time
- The passengers got on the bus one by one and showed their tickets to the driver.
- I went through the customer complaints one by one and replied to each of them.
- One by one, my school friends moved to bigger cities for work.
What changed
First versionThe students entered the exam room one after one and showed their ID cards.
RevisionThe students entered the exam room one by one and showed their ID cards.
One after one might look like a natural pattern, but English doesn't have it. The phrases are one by one and one after another; both mean that each one follows the previous one.
One at a time stresses a limit: no more than one at once. You often hear it in rules and requests: Please come in one at a time. One by one usually describes how things happened: One by one, the lights in the office went out.
Your turn
The doctor called us into her office ___, so each of us waited to hear our name.
Show the answer
one by one — The doctor called the patients in turn, each one separately, so one by one. Step by step is about the stages of one task, not about people going in one after another.
Describe a day at work or university when a lot of similar tasks piled up. Say how many there were and what you did with them.
On Monday I had five urgent emails, so I answered them one by one before starting the weekly report.
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