Co-signatory is a word that typically refers to a person who signs a document with another person, usually for an agreement or a contract. However, there are other words that can be used as a synonym for co-signatory. These include signatory, co-signer, co-obligor, co-debtor, and co-contractor. Signatory is the most commonly used synonym for co-signatory and refers to a person who signs or has signed a document. Co-signer is also widely used and refers to a person who signs a document with another person to guarantee payment. Co-obligor and co-debtor are used mostly in legal documents and refer to persons who share a debt obligation with another. Co-contractor, on the other hand, is a synonym used to describe persons who work together on a project or a contract.