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mind (one's) p's and q's
To be polite and display good manners. Be sure to mind your p's and q's when you visit your aunt this weekend! The mother reminded her young children to mind their p's and q's when they went over to their friends' houses for play dates.
mind your Ps and Qsbe careful to behave well and avoid giving offence.
Various suggestions have been made concerning the significance of P and Q . One obvious one is that a child learning to read or write might have difficulty in distinguishing between the two tailed letters p and q . Another is that printers had to be very careful not to confuse the two letters when setting type.