Convention

"A pattern (for instance, the 14-line poem, or sonnet) or motif (for instance, the bumbling police officer in detective fiction) or other device occurring so often that it is taken for granted. Thus it is a convention that actors in a performance of Julius Caesar are understood to be speaking Latin, though in fact they are speaking Engish. Similarly, the soliloquy (a character alone on the stage speaks his or her thoughts aloud) is a convention, for in real life sane people do not talk aloud to themselves." (Literature for Composition, 1074).