This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
Alterations are to be classified upon two different bases: they may as to their effect upon the instrument be material or immaterial; and as to the purpose with which they are made they may be fraudulent or innocent. A material alteration is one which in some way changes the legal effect of the instrument.1
Precedents for determining whether specific changes are material or immaterial must be used with great caution, since it is only by determining the legal effect of the instrument before the alteration and after the alteration that it is possible to decide whether the change is material or not. As will be seen from comparing examples of material and immaterial alterations, a specific change may be a material alteration in one contract and an immaterial alteration in another. The rules here given are merely general statements, subject to the exception of particular forms of contract.