The word "契约" is formed through the conceptual blending of two ancient characters: "契", which originally depicted the carving of tallies or notches into wood or bamboo to create a physical record of an agreement, and "约", whose core meaning is to bind or restrain, as with a silk thread, representing the obligation and binding nature of a promise. Together, they combine the idea of a tangible, recorded pledge ("契") with the concept of a mutually binding restraint ("约") to perfectly encapsulate the modern notion of a formal, reciprocal contract, an agreement that is both documented and morally or legally obligatory for all parties involved.