(n.) The state or quality of being isochronous.

accession accessory accordance addenda addendum additament addition additive additory additum adjunct adjuvant agreement alliance alto annex annexation appanage appendage appendant appurtenance appurtenant association attachment augment augmentation baritone bass basso continuo basso ostinato bassus cahoots canto cantus cantus figuratus cantus planus co-working coaction coda coetaneity coetaneousness coevalneity coevalness coexistence coincidence collaboration collectivity collusion combination combined effort complement concert concerted action concomitance concomitant concordance concourse concurrence confluence conjunction consilience conspiracy contemporaneity contemporaneousness continuation continuo contralto cooperation corollary correspondence descant drone extension extrapolation figured bass fixture ground bass increase increment isochronism junction line offshoot parasitism part pendant plain chant plain song prick song reinforcement saprophytism side effect side issue simultaneity soprano supplement symbiosis synchronism synchronization synergy tailpiece tenor thorough bass treble undergirding undersong union unison united action voice voice part