one by one

one by one idiom
Also, one at a time. Individually in succession, as in The ducklings jumped into the pond one by one, or One at a time they went into the office. Formerly also put as one and one and one after one, this idiom dates from about a.d. 1000.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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