U.S. Supreme Court
Saltmarsh v. Tuthill, 54 U.S. 13 How. 229 229 (1851)
Saltmarsh v. Tuthill
54 U.S. (13 How.) 229
In a suit by the endorsee against the endorser of a bill, where the defense was usury, the drawer and drawee were incompetent witnesses when offered to prove certain facts which, when taken in conjunction with certain other facts, to be proved by other witnesses, would invalidate the instrument.
Being incompetent witnesses to establish the whole defense, they are also incompetent to establish a part.
The only question was one of evidence, which is fully explained in the opinion of the court.