A rationale is a liturgical vestment worn by clergy, in particular by Bishops, in the Roman Catholic Church which uses full vestments. It is humeral ornament, a counterpart to the Pallium, and is worn over the chasuble. It is a large clasp, made of precious metal and ornamented with diamonds, whose edges are adorned with small bells. The word is more commonly used to mean an explanation of the basis or fundamental reasons for something.
During the Middle Ages it was worn by the Bishops of Würzburg, Ratisbon, Eichstätt, Naumburg, Halberstadt, Paderborn, Minden, Speier, Metz, Augsburg, Prague, Olmutz, Liège, and Toul. Its use largely died out in the 13th century, although there is evidence that it was worn at Reims until the 16th century. Some rationales can be found preserved at Bamberg and Ratisbon. The earliest pictures of rationales that exist are two pictures of Bishop Sigebert of Minden, a miniature and an ivory tablet, which were both incorporated in a Mass Ordo belonging to the Bishop.
The only Bishops that wear rationales in the 21st century are the Bishops of Eichstätt, Paderborn, Toul, and Kraków. The modern rationale is a humeral collar, ornamented in the front and back with appendages.
The rationale is named after the breast ornament of a Jewish high-priest, which was a wooden brooch, overlaid with enamelled metal, fastened high up on the breast of chasubles that had no central orphreys.