Lissajous figure, alternation, back-and-forth, balance, battledore and shuttlecock, boomerang, coequality, comeback, coming and going, communion, community, commutation, cooperation, corelation, correlation, correlativism, correlativity, correspondence, counter, counterblast, counterblow, counterchange, counterstroke, cross fire, ebb and flow, equilibrium, equipollence, equivalence, exchange, flux and reflux, give-and-take, interchange, intermutation, interplay, inverse proportion, inverse ratio, inverse relationship, lex talionis, measure for measure, mutual admiration, mutual support, mutual transfer, mutuality, permutation, proportionality, quid pro quo, reciprocality, reciprocity, recoil, relativity, reply, retaliation, retort, return, rhythmic play, seesaw, seesawing, sine wave, something for something, symmetry, teeter-totter, teeter-tottering, teeter, teetering, tit for tat, to-and-fro, tottering, transposal, transposition, ups and downs, wigwag