Define the contract the addendum will change. For example, if your original contract is an employment contract made between Business X and John Doe dated June 30, 2014, write as the first paragraph of your addendum: "This amendment is made between Business X and John Doe, parties to the employment contract dated June 30 2014 ("the Original Agreement")." An addendum contract is an agreed-upon complement to the initial agreement made by all sides. It specifies the parts and meanings including the particular conditions, provisions, etc., to be altered in the initial agreement, but otherwise leaves the rest of the contract in absolute force and effect.