 Some types may be refrigerated or frozen and then precooked and then reheated in an oven or microwave oven as necessary before serving.If there is an extended period between when guests arrive and when the meal is eaten, for example during a cocktail hour, these might serve the purpose of sustaining guests during the wait, in the same way that apéritifs are served as a drink before meals.It is also an unwritten rule that the dishes served as hors d'oeuvres do not give any clue to the main meal.