Required by the reigning code of taste, etiquette, or fashion. The obligation here is social, not legal, enforced by approval and embarrassment rather than any written statute. There is often something wry in its use, an acknowledgment that the tribunal is made of eyes.

From French de rigueur "of strictness," from rigueur "strictness, severity." The phrase entered English with its irony intact: a strictness that governs trifles, and trifles that govern lives.