decorum (n.)

1560s, "that which is proper or fitting in a literary or artistic composition;" 1580s, "propriety of speech, behavior, or dress; formal politeness," from Latin decorum "that which is seemly," noun use of neuter of adjective decorus "fit, proper," from decor"beauty, elegance, charm, grace, ornament," from decus (genitive decoris) "an ornament," from PIE root *dek- "to take, accept" (on the notion of "to add grace").