Everyday Phrases That Come from Printing - mind your p's and q's
This phrase means being on one’s best behaviour in British English, and paying close attention in American English. Both versions make sense coming from the print shop. Setting type means placing each individual letter in backward, so that when the inked type is pressed into paper, the mirror image reads the right way forward.
This required a certain amount of focus from the workers who set the type (known as “compositors”), especially when it came to letters that look like mirror images of each other. In older type cases, each letter was kept in a segregated section to be picked out by the compositor setting the type. The lowercase p’s and q’s are right next to each other, just begging to be mixed up. That’s why it’s “mind your p’s and q’s,” not “mind your b’s and d’s,” which are not neighbours in the type case.