Arris is an architectural term that describes the sharp edge formed by the intersection of two surfaces, such as the corner of a masonry unit; the junction between two planes of plaster or any intersection of divergent architectural details. Also the raised edges which separate the flutings in a Doric column.
The origin is from the Latin 'arista' meaning the beard of an ear of grain or the bone of a fish. See also arête.
An arris rail is a structural element, whose cross sectional area has two sides forming a right angled triangle. Arris rails are usually made of wood, and are manufactured by cutting a 45° chamfer from both edges of a timber, at the end of the boards. They are used for structures which require joining two timbers at right angles; for example, connecting a wood post and beam. Another common use is for the horizontal rails of timber fences, since the diagonal edges prevent water from collecting on top of the wood and thus rotting the timber. It also adds an element of security, since the fence is harder to climb.