The five Girih tiles (from the Persian word meaning "knot") are the fundamental patterns of Islamic art, in use since the 13th century. A tile has every edge of the same length, interior angles from the set 72°, 108°, 144°, or 216°, and a polygonal line that meets the sides at the midpoints; when two tiles touch on a common side, the two line segments form a straight angle. Girih tiles closely resemble Penrose tiles.