In classical rhetoric, decorum is the use of a style that is appropriate to a subject, situation, speaker, and audience.
According to Cicero's discussion of decorum in De Oratore (see below), the grand and important theme should be treated in a dignified and noble style, the humble or trivial theme in a less exalted manner.
Examples and Observations
"Decorum is not simply found everywhere; it is the quality whereby speech and thought, wisdom and performance, art and morality, assertion and deference, and many other elements of action intersect. The concept underwrites Cicero's alignment of the plain, middle, and elevated oratorical styles with the three main functions of informing, pleasing, and motivating an audience, which in turn extends rhetorical theory across a wide range of human affairs." (Robert Hariman, "Decorum." Encyclopedia of Rhetoric. Oxford University Press, 2001)
Aristotle on Aptness of Language
"Your language will be appropriate if it expresses emotion and character, and if it corresponds to its subject. 'Correspondence to subject' means that we must neither speak casually about weighty matters, nor solemnly about trivial ones; nor must we add ornamental epithets to commonplace nouns, or the effect will be comic… To express emotion, you will employ the language of anger in speaking of outrage; the language of disgust and discreet reluctance to utter a word when speaking of impiety or foulness; the language of exultation for a tale of glory, and that of humiliation for a tale of pity and so on in all other cases.
"This aptness of language is one thing that makes people believe in the truth of your story: their minds draw the false conclusion that you are to be trusted from the fact that others behave as you do when things are as you describe them; and therefore they take your story to be true, whether it is so or not."
Cicero on Decorum
"For the same style and the same thoughts must not be used in portraying every condition in life, or every rank, position, or age, and in fact a similar distinction must be made in respect to place, time, and audience. The universal rule, in oratory as in life, is to consider propriety. This depends on the subject under discussion and the character of both the speaker and the audience…
"This, indeed, is the form of wisdom that the orator must especially employ--to adapt himself to occasions and persons. In my opinion, one must not speak in the same style at all times, nor before all people, nor against all opponents, not in defence of all clients, not in partnership with all advocates. He, therefore, will be eloquent who can adapt his speech to fit all conceivable circumstances."
(Cicero, De Oratore)
"In opposition to Cicero, whose ideal was to 'discuss commonplace matters simply, lofty subjects impressively, and topics ranging between in a tempered style,' Saint Augustine defends the manner of the Christian gospels, which sometimes treat the smallest or most trivial matters in an urgent, demanding high style. Erich Auerbach in Mimesis, 1946 sees in Augustine's emphasis the invention of a new kind of decorum opposed to that of the classical theorists, one oriented by its lofty rhetorical purpose rather than its low or common subject matter. It is only the aim of the Christian speaker--to teach, admonish, lament--that can tell him what sort of style to employ. According to Auerbach, this admission of the most humble aspects of daily life into the precincts of Christian moral instruction has a momentous effect on literary style, generating what we now call realism." (David Mikics, A New Handbook of Literary Terms. Yale University Press, 2007)
Decorum in Elizabethan Prose
"From Quintilian and his English exponents (plus, it must not be forgotten, their inheritance of normal speech patterns) the Elizabethans at the end of the 16th century learned one of their major prose styles. Thomas Wilson had preached the Renaissance doctrine of decorum: the prose must fit the subject and the level at which it is written. Words and sentence pattern must be 'apt and agreeable.' These may vary from the condensed native maxim like 'Enough is as good as a feast' (he recommends Heywood's proverbs which had recently appeared in print) to the elaborate or 'exonerated' sentences adorned with all the 'colours of rhetoric.' Exoneration opened the way--and Wilson provided full examples--for new sentence structures with 'egall members' (the balanced antithetical sentence), 'gradation' and 'progression' (the paratactic cumulation of short main clauses leading to a climax), 'contrarietie' (antithesis of opposites, as in 'To his friend he is churlish, to his foe he is gentle'), the series of sentences with 'like endings' or with 'repetition' (like opening words), plus the verbal metaphors, the longer 'similitudes,' and the whole gallery of 'tropes,"schemes,' and 'figures of speech' of the last few decades of the 16th century." (Ian A. Gordon, The Movement of English Prose. Indiana University Press, 1966)