Pointed arch

A pointed arch is formed by two arcs, each drawn from its own centre, meeting at a point at the top. Unlike a round arch, its crown is sharp rather than curved. Depending on how far apart the centres are, it can be tall and steep or low and wide. It is a basic element of Islamic architecture, of Anatolian Seljuk and Ottoman buildings, and of Gothic architecture in Europe.

How the load travels

An arch carries the load above it along its curves down to the springing points on either side, and in doing so creates horizontal thrust as well as vertical load. The geometry of a pointed arch keeps the line of thrust steeper than in a round arch. For the same span it pushes outward less and can stand on slimmer supports. It can also bring openings of different widths up to the same crown height, which makes vaulting and dome systems much easier to resolve.

• Springer: the first stone where the arch begins
• Voussoirs: the wedge-shaped stones along the curve
• Crown: where the two arcs meet at the top

Pointed arch versus round arch

A round arch is always half as high as it is wide. A pointed arch can be made taller or lower by moving its centres. That flexibility made it a powerful tool in historic buildings, both structurally and for proportion.

Survey and 3D modeling

A survey tries to establish the span, rise and the centres of both arcs, which reveals the geometric rule used to set out the arch. In historic buildings the two arcs are often not symmetrical. Fitting two arcs through measured points in 3D gives a more accurate result than an idealized arch.