1
general::
Phrase(s): *in (a) single file
Fig. lined up, one behind the other; in a line, one person or one thing wide. (*Typically: be ~; get [into] ~; march ~; walk ~.) • Have you ever seen ducks walking in single file? • Please get into single file. • Please march in single file.
McGrawhill's American Idioms And Phrasal Verbs
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general::
see SINGLE FILE., Also, in Indian file. Aligned one behind the other, as in We have to bike in single file here, or The children were told to march in Indian file. Both usages are associated with military formations; the first term was first recorded in 1670; the variant, alluding to the usual marching order of Native Americans, was first recorded in 1758.
American Heritage Idioms