Clandestinity is a diriment impediment in the canon law of the Roman Catholic Church. It requires the presence of witnesses to the marriage vows, one of which must be a priest, in order for the marriage to be valid. It was promulgated in the 16th century by the Council of Trent in the decree called Tametsi. Prior that time, an unwitnessed exchange of marriage vows was deplored but valid, and the decree was enforced only in those regions where it could be proclaimed in the vernacular