A signature is an act of identity. To cosign is an act of shared trust.
It is the second key turning in the lock, the partner's verification, the consensus that transforms a single claim into a validated reality. It is the principle that no single point of failure should ever be enough.
Cosign is the landmark for the technology of mutual assurance. For the multi-sig wallet, the collaborative platform, and the new finance built not on one, but on the agreement of many.