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An oath which was administered to a party to an action who testified in his own behalf. The rule of the law courts required two witnesses to supply full proof of a single fact, and hence it was held that the testimony of one witness supplied only half proof. And where one witness had testified to a single fact in a party's behalf, he was-contrary to the general rule-in order to make full proof, permitted to testify in his own behalf, and it was then that the suppletory oath was administered to him.