Rank and file
The ordinary members of a group; as opposed to the group leadership.
Rank and file now refers to the ordinary members of any group but it originated as a military term. The rows and columns of soldiers, drawn up for drill and not including officers, were called 'ranks' and 'files'. This usage dates back to the 16th century and an early example of it in print is found in Robert Barret's The theorike [sic] and practike of moderne warres, 1598:
To learne to keepe his ranke and file orderly.
The expression is now often shortened just to 'the ranks'.
It wasn't until the 19th century that 'rank and file' began being used to refer to any other groups. The first of these were political groupings, especially members of trades unions. An example of this was given is in an address from a representative of the American Federation of Labor, reported in The International Steam Engineer magazine, 1818:
"If we could only get the great rank and file of the members to recognise their personal obligation."
Chess is often described as analogous to warfare and has inherited military parlance in that the rows and columns of pieces are called ranks and files.