Complementary

The quality of a relation between parts of contrasting identity or that behave so that the moves by one fit the moves by the other(s). E.g. the economic transfer of goods requires a buyer and a seller, neither exists without the other; a court of law requires a judge, a plaintiff, a respondent and a stenographer, neither can enact the proceedings without the other's participation each complementing the other in forming the whole. Examples of other complementarities are subject/object, text/context, autonomy/control, stability/change, being/becoming, pleroma/creatura, self/other, all of which involve circular definitions of each other. (Krippendorff )