A: The act of indemnifying (co-signing) involves a promise to pay another person's debt arising out of a contract if that person fails to do so. Many realtors and landlords require a co-signer for college students, people with bad credit or people whose income is less than a certain, low multiple of the amount of rent. Other loans typically involving a co-signer are motor vehicle purchase money loans and mortgages. The statute of frauds existing in most states of the United States requires that any such agreement be in writing and signed by the co-signer in order to be enforceable in a court of law. A co-signer is also known as a surety. The legal act or instrument of co-signing is also called a guarantee.