To mind your Ps and Qs means to be polite and to mind your manners.
The origin of this phrase is unclear, there are several possible explanations, all equally plausible:
- inexperienced printers, back in the days of letterpress and movable type, would have to be particularly careful when selecting the letters p and q as they look very similar, and the reverse letters required for printing are even more confusing
- children, on being taught politeness would be encouraged to remember their ‘pleases and thank yous’
- inns and taverns would often run bar tabs for customers (often chalked on a piece of slate – hence ‘putting it on the slate’) – these bar tabs would comprise a list of p’s – representing pints or q’s representing quarts. As a quart contains two pints, the difference could be substantial, so bar staff would be encouraged to mind their p’s and q’s.
