symmetrical (adj.)

1560s, "relation of parts, proportion," from French symmétrie (16c.) and directly from Latin symmetria, from Greek symmetria "agreement in dimensions, due proportion, arrangement," from symmetros "having a common measure, even, proportionate," from assimilated form of syn- "together" (see syn-) + metron "measure" (from PIE root *me- (2) "to measure"). Meaning "harmonic arrangement of parts" first recorded 1590s.

compound adjectival word-forming element, usually interchangeable with -ic but sometimes with specialized sense (such as historic/historical, politic/political), Middle English, from Late Latin -icalis, from Latin -icus + -alis (see -al (1)). Probably it was needed because the forms in -ic often took on a noun sense (for example physic). Forms in -ical tend to be attested earlier in English than their twins in -ic.

"divisible into exactly similar halves by more than one plane," by 1875; see poly- "many" + symmetrical. Related: Polysymmetry; polysymmetrically.