Note to Romeo and Juliet, 2.4.25: "of the first and second cause"

the first and second cause: obligatory reasons to enter into a duel. Duelling academies taught codes of duelling which listed causes—insufferable insults or actions—for a quarrel. Mercutio's attitude is that such duelling codes are simply excuses for wannabe tough guys, such as Tybalt, to pick a fight.

Shakespeare also mocks duelling codes in As You Like It by having Touchstone the fool describe how he almost fought a duel, but escaped actual combat by means of an "if."