Arris

In architecture, an arris is the sharp edge formed by the intersection of two surfaces, such as the corner of a masonry unit;[1] the edge of a timber in timber framing; the junction between two planes of plaster or any intersection of divergent architectural details.

The origin of the term arris is from the Latin arista, meaning the beard or the ear of grain or the bone of a fish.

An arris rail is a structural element, whose cross section is a 45 degree isosceles right angled triangle.

Arris rails are usually made of wood, and are manufactured by cutting a length of square-section timber lengthwise diagonally.

In the 16th century, the Italian architect Cosimo Bartoli alongside various other writers reference the arris in L'architettura.