The word addendum comes from the Latin word addere, meaning “to add.” An addendum is an addition to a piece of writing — a document, publication, or book, for example — that modifies, clarifies or adds to a specific part of the original written work. One of the most important aspects of an addendum is that it is added before the document is executed. Once added, it becomes part of the original document.

Addendums are often used with legal contracts to add or specify conditions to the contract. Real estate transactions commonly use addendums to specify financial or home inspection requirements for securing a mortgage. A renter may sign an addendum to the lease agreement if special rules apply to him, such as conditions for allowing him to have a pet. In construction, an addendum may be used during the initial bidding to further clarify what will be provided via the contract.

When an addendum is added to a contract, it is normally signed in the presence of a witness, just as the original contract is. It may even be notarized in some cases. By signing, the parties involved indicate that they agree to the stipulations as outlined. In other cases, a signature may not be required, as the document is not a contract. In architecture, for example, it might contain drawings that clarify or modify a project. A book may contain an addendum that further explains some part of it, such as an appendix.

Sometimes an addendum is confused with an amendment, but they are two different types of additions. The primary difference is that an addendum is added before the document is executed, while an amendment is added after the document is executed. The addendum is agreed upon, signed if it is part of a contract, and presented as part of the original document. An amendment may be added at any time after the original document has been presented, and is usually used to make changes to the original document. Therefore, the two terms have specific proper uses and are not interchangeable.