E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Rank and File.
Soldiers of any grade below that of lance-srgeant are so called, collectively, in military phraseology, and any two soldiers of such grade are spoken of as a file; thus, 100 rank and file would equal 50 file, that is, 50 men standing behind each other in a row. No soldier ever talks of files in the plural, or about a file of fours. As there are two in a rank, there is a left file and a right file; and men may move in single file or in double file. A line of soldiers drawn up side by side or abreast is a rank.