These words describe actions that are judged to be inescapably necessary. Compulsory suggests that someone in authority has imposed a course of action that may not be departed from compulsory attendance of classes. It may further suggest that the compulsory ruling will be enforcing, by coercion if necessary, and that violators will be punished: compulsory blacklisting. Mandatory is like compulsory in suggesting an imposed rule: mandatory silence in the library. In contrast, it is milder than compulsory in that it stops short of suggesting coercion and punishment as methods of enforcing the ruling.

Binding also describes a ruling handed down by an authority, but it may specifically suggest a one-time decision concerning a specific dispute between two parties who agree beforehand to abide by the results: binding arbitration. It may also suggest that earlier, more tentative rulings have been submitted to a final authority.

• The teacher’s compulsory ruling against wearing Bermuda shorts to class was referred to the principal, whose decision would be binding .

Obligatory is like binding in its legal sense. In a more general use, however, it suggests something expected or made necessary by morality or conscience, rather than by a higher authority as in the previous words: the obligatory chaperone at teenage parties. As such, its force is more what should be done than what is or must be done: attendance at tutorial classes is obligatory . Imperative refers to a necessity dictated by circumstance rather than by an authority or a code of morality: imperative to get food and water before sundown. If used to refer to an imposed rule, it is more like mandatory than compulsory .

De rigueur is close to obligatory , but is milder in referring to those acts or observance considered necessary according to etiquette or required by good form; it usually appears only as a predicate adjective: an unwritten rule that made evening dress de rigueur for attending her soirées.