A jack arch is a structural element in

masonry Masonry is the craft of building a structure with brick, stone, or similar material, including mortar plastering which are often laid in, bound, and pasted together by mortar (masonry), mortar. The term ''masonry'' can also refer to the buildin ...

construction that provides support at openings in the masonry. Alternate names are lintel arch, flat arch and straight arch. Unlike regular

arch An arch is a curved vertical structure spanning an open space underneath it. Arches may support the load above them, or they may perform a purely decorative role. As a decorative element, the arch dates back to the 4th millennium BC, but stru ...

es, jack arches are not curved in form. Instead, they are flat in profile and are used under the same circumstances as

lintel A lintel or lintol is a type of beam (a horizontal structural element) that spans openings such as portals, doors, windows and fireplaces. It can be a decorative architectural element, or a combined ornamented/structural item. In the case ...

s. Unlike lintels, which are subject to bending stress, jack arches are composed of individual masonry elements cut or formed into a wedge shape that efficiently uses the compressive strength of the masonry in the same manner as a regular

. Like regular arches, jack arches require a mass of masonry to either side to absorb the considerable lateral thrust created by the jack arch. Jack arches have the advantage of being constructed from smaller pieces of material that can be handled by individuals, as opposed to lintels which must necessarily be monolithic and which must be oversized unless reinforced by other means. In small-scale brick masonry projects, jack arches are typically sawn from an appropriately sized fired-clay lintel, giving a more precise and consistent joint width than field-sawn shapes. There is considerable scope for incorporation of decorative patterns and elements into jack arches. Keystones, stepped or arched top profiles, and

polychrome Polychrome is the "practice of decorating architectural elements, sculpture, etc., in a variety of colors." The term is used to refer to certain styles of architecture, pottery, or sculpture in multiple colors. When looking at artworks and ...

or contrasting colors and materials may all be used to create the desired effect. One of the earliest jack arches is in the Deyrü'z-Zafaran Monastery in eastern Turkey, near

Mardin Mardin (; ; romanized: ''Mārdīn''; ; ) is a city and seat of the Artuklu District of Mardin Province in Turkey. It is known for the Artuqids, Artuqid architecture of its old city, and for its strategic location on a rocky hill near the Tigris ...

. The floor of the chapel (and the roof of the crypt) is formed from a jack arch and is said to have been constructed in the centuries BC.

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* * * * Arches and vaults Masonry {{Architecturalelement-stub