Roof Boss, 1400s. Northern Spain, 15th century. Limestone with traces of polychromy; overall: 76.2 x 76.2 cm (30 x 30 in.). In medieval architecture, a "boss" refers to an ornamentally carved, often painted and gilded, keystone. It was placed at the intersection of ribs in a vaulted roof in order to lock its members into position. Noteworthy examples are found in many of Europe's monastic cloisters and cathedrals. This example and the other shown nearby are reportedly from the vaulted ceiling of a monastery in Northern Spain. Their principal decorative motif consists of a spray of acanthus leaves.