An agreement, contract, conveyance, sale, or other transfer of real estate, or an interest in it, usually must be set forth in writing and signed. The writing requirement is found in a body of law, sometimes referred to as the "statute of frauds." This law makes oral or verbal transfers of real estate unenforceable. The written document setting forth the terms of the sale or transfer is referred to as "an instrument." Simply put, an instrument is a written document. To be enforceable, a sale or transfer of real estate, including an encumbrance such as a mortgage, must be evidenced by a properly executed instrument. Some examples of instruments affecting real estate include: contracts, deeds, leases, mortgages or deeds of trust.

A possible exception to the writing requirement occurs where one or more of the parties to the agreement have partly performed their obligations under the contract and strict enforcement of the writing requirement would be inequitable or unfair. Another exception occurs where one or both sides to an oral agreement admit the terms of the contract and agree to put it in writing or agree that it will be enforced. Please see specific state for details and/or differences.

Black's Law Dictionary, Fifth Edition.