congruity n.

word-forming element meaning "together, with," sometimes merely intensive; it is the form of com- used in Latin before consonants except -b-, -p-, -l-, -m-, or -r-. In native English formations (such as costar), co- tends to be used where Latin would use con-.

early 15c., "suitable, proper, harmoniously joined or related," from Latin congruentem (nominative congruens) "agreeing, fit, suitable," present participle of congruere "agree, correspond with," literally "to come together," from assimilated form of com "with, together" (see con-) + a lost verb *gruere, *ruere "fall, rush," which de Vaan traces to a PIE *ghr(e)uho- "to rush in." 

Geometry sense, "capable of being superposed," is attested by 1706. Related: Congruently.

1530s, "quality of being incongruent," from French incongruité (16c.) or directly from Medieval Latin incongruitas, from Latin in- "not" (see in- (1)) + congruitas (see congruity). Meaning "that which is incongruent, an example of incongruency" is from c. 1600.