Material alteration vitiates an instrument's original legal position when made without consent, akin to cancelling the deed. A material alteration changes the rights, liabilities, or legal position created by an instrument, or its legal effect, or renders an originally uncertain provision certain; made without the consent of the bound party it operates like cancellation. By contrast, alterations that do not vary the instrument's legal effect-such as filling in an execution date or inserting occupier names where parties and property are already ascertained-are non-material and do not affect validity.