Decorum

Decorum (from the Latin: "right, proper") was a principle of classical rhetoric, poetry and theatrical theory that was about the fitness or otherwise of a style to a theatrical subject. The concept of decorum is also applied to prescribed limits of appropriate social behavior within set situations.

In classical rhetoric and poetic theory, decorum designates the appropriateness of style to subject. Both Aristotle (in, for example, his Poetics) and Horace (in his Ars Poetica) discussed the importance of appropriate style in epic, tragedy, comedy, etc. Horace says, for example: "A comic subject is not susceptible of treatment in a tragic style, and similarly the banquet of Thyestes cannot be fitly described in the strains of everyday life or in those that approach the tone of comedy. Let each of these styles be kept to the role properly allotted to it."

Hellenistic and Latin rhetors divided style into: the grand style, the middle style and the low (or plain) style; certain types of vocabulary and diction were considered appropriate for each stylistic level. A discussion of this division of styles was set out in the pseudo-Ciceronian Rhetorica ad Herennium. Modeled on Virgil's three-part literary career (Bucolics, Georgics, Aeneid), ancient, medieval and Renaissance theorists often linked each style to a specific genre: epic (high style), didactic (middle style) and pastoral (plain style). In the Middle Ages, this concept was called "Virgil's wheel". For stylistic purists, the mixing of styles within a work was considered inappropriate, and a consistent use of the high style was mandated for the epic. However, stylistic diversity had been a hallmark of classical epic (as seen in the inclusion of comic and/or erotic scenes in the epics of Virgil or Homer). Poetry, perhaps more than any other literary form, usually expressed words or phrases that were not current in ordinary conversation, characterized as poetic diction.

...
Wikipedia