A central rhetorical principle requiring one's words and subject matter be aptly fit to each other, to the circumstances and occasion (kairos), the audience, and the speaker.

Though initially just one of several virtues of style ("aptum"), decorum has become a governing concept for all of rhetoric. Essentially, if one's ideas are appropriately embodied and presented (thereby observing decorum), then one's speech will be effective. Conversely, rhetorical vices are breaches of some sort of decorum.

Decorum invokes a range of social, linguistic, aesthetic, and ethical proprieties for both the creators and critics of speech or writing. Each of these must be balanced against each other strategically in order to be successful in understanding or creating discourse.

Besides being an overarching principle of moderation and aptness, decorum has been a controlling principle in correlating certain rhetorical genres or strategies to certain circumstances. Aristotle describes each of the branches of oratory as being appropriate to judicial, legislative, or epideictic occasions and to specific time periods (past, future, and present, respectively). The concept of stasis included a procedure for discovering and developing arguments appropriate to given circumstances. Cicero followed the principle of decorum in assigning an appropriate level of style to distinct rhetorical purposes. Throughout rhetoric, decorum structures the pedagogy and procedures of this discipline as much as it governs the overall uses of language.