Cautio
Although in Justinian's time there were no contracts deriving their force from the fact that they were in writing, written acknowledgments of indebtedness were in use. Thus a man who gave a written acknowledgment of a loan was estopped thereby after the lapse of two years the period having been reduced from five years by Justinian to deny that he had received the money.
Such an acknowledgment — known as a cautio — was not a contract, it did not create a debt, but was merely evidence that a debt existed. This evidence was prima facie only until two years had elapsed, when it became conclusive. The cautio seems to have grown out of the chirographum, but with this radical difference, the chirographum created a debt while the cautio was merely evidence of a debt.