A practice agreed rather than required. A convention is not a rule, and treating it as one is a common error.
Latin conventio (a meeting, agreement), from convenire (to come together, agree), from com- (together) + venire (to come); a practice reached by people coming together and agreeing, never imposed by a single will.
A practice agreed rather than required. Treating a convention as a rule is a common and understandable error, but not a correct one.