BBB Etymology - Mind Your Ps and Qs

This expression comes from an old British bartender's call. When arguments or fights would break out, the barkeeps would call for everyone in the bar to mind their pints and quarts (of ale). Hence, mind your own business.

Or maybe:

• Advice to a child learning its letters to be careful not to mix up the handwritten lower-case letters p and q.
• Similar advice to a printer’s apprentice, for whom the backward-facing metal type letters would be especially confusing.
• Jocular, or perhaps deadly serious, advice to a barman not to confuse the letters p and q on the tally slate, on which the letters stood for the pints and quarts consumed “on tick” by the patrons.
• An abbreviation of mind your please’s and thank-you’s.
• Instructions from a French dancing master to be sure to perform the dance figures pieds and queues accurately.
• An admonishment to seamen not to soil their navy pea-jackets with their tarred queues, that is, their pigtails.