Two structures are complements if they fit together or ‘dovetail’ with each other. A key matches a lock; a glove properly fits a hand; a photographic negative produces a corresponding print. The two antiparallel strands of DNA are complements. One strand serves as a template for the synthesis of a complement. Substrates for enzymes fit into active sites. The shape of the bottom partof a foot can be reconstructed from a mold of the footprint.
