As applied in modern practice, the "best evidence rule" means little more than that the contents of a written instrument must be proved by the production of the written instrument itself, unless its absence is satisfactorily explained. The rule is not confined to matters which the law requires to be in writing, but extends to all writings, except notices, the contents of which may be proved by a copy, without accounting for the non-production of the original notice.

It is also a well established rule of law that no evidence will be received to alter, add to, or vary the terms of a written instrument.