Extract

An oath (iusiurandum) was used in several ways in the stage before the magistrate (in iure) in Classical Roman civil proceedings.

(1) In almost every action either party might exact from the other, on pain of losing the case, an oath that he was proceeding in good faith (iusiurandum calumniae). Justinian made this oath compulsory for both parties and for their representatives (see justinian's codification).

(2) In a few actions only the plaintiff might invite the defendant to swear to the validity of his claim (deferre iusiurandum); the defendant might then either swear and win the case, or refuse and lose, or invite the plaintiff to swear instead, with the same alternatives before him (referre iusiurandum), or, finally, might invite the plaintiff to swear a iusiurandum calumniae.

(3) Either party in any action (or at any stage of any dispute) might invite the other to swear an oath of this kind; the other might either swear and win or refuse with impunity.