The most prevalent types were mosaics and icons. Byzantine church architecture is often central rather than longitudinal, with pendentives supporting the center dome (surrounded by clusters of lesser or semi-domes). The structure is made of bricks laid in ornamental patterns and mortar. Walls were generally plain except for some coloration at the top of the wall behind the vaults. Mosaics were used as flooring, wall decorations, and ceilings. They could also be used as instruments: a mosaic bow played by squeezing two fingers against its side produced a twangy sound similar to that of a violin string. Mosaic bows were made from the backbones of goats or sheep stretched over a frame and covered with leather or cloth skin. The best-known mosaic artist of Byzantine times was Paul of Damascus, who worked in Constantinople between 867 and 908. He designed and painted many religious images in collaboration with other artists. Icons are famous ancient Christian paintings on wood or panel. Icons usually show Christ, the Virgin Mary, saints, angels, or other biblical figures. Although icons originated in Eastern Christianity, they became popular throughout Byzantium. Icons were placed in church sanctuaries to protect those who entered the room from sin, illness, or evil spirits. People would pray to them for help. Byzantine art had great influence on medieval and Renaissance art.