P's and q's

[Sarah]

From an old printers axiom. Back in the early days of printing presses, each line of text had to be set up one letter at a time. Since the letters in the press were reversed (so theyd print forward), the printmaker (or typographer) needed to be careful not to confuse one letter for the other.
Reminding someone to watch his ps and qs means to pay attention to the details.