A Treatise on Painting/Chapter 149
Chap. CXLIX.—Of Decorum.
Observe decorum in every thing you represent, that is, fitness of action, dress, and situation, according to the dignity or meanness of the subject to be represented. Be careful that a king, for instance, be grave and majestic in his countenance and dress; that the place be well decorated; and that his attendants, or the by-standers, express reverence and admiration, and appear as noble, in dresses suitable to a royal court.
On the contrary, in the representation of a mean subject, let the figures appear low and despicable; those about them with similar countenances, and actions, denoting base and presumptuous minds, and meanly clad. In short, in both cases, the parts must correspond with the general sentiment of the composition.
The motions of old age should not be similar to those of youth; those of a woman to those of a man; nor should the latter be the same as those of a boy.