The printing industry gives us other popular phrases, such as "mind your 'p's and 'q's." The moveable block type had the letters in reverse so they would read correctly when imprinted on paper. Apprentices had to remove the type from the pages and return the blocks to their upper and lower cases.
Each drawer in the case held a different size of letters, and each drawer was divided into compartments (called sorts) for each letter. The letters 'p' and 'q' could easily be mistaken, so the master printer would advise their apprentices to mind their 'p's and 'q's.