Horseshoe arch

A horseshoe arch carries its curve beyond a semicircle, so that it turns back inward below the springing points and narrows the bottom of the opening. The shape recalls a horseshoe. The crown can be round or slightly pointed. Andalusian architecture, above all the Great Mosque of Córdoba, provides the best-known examples, and the form is widespread in North Africa and elsewhere in the Islamic world.

Role in the building

Structurally a horseshoe arch behaves much like a round arch, carrying load along its curves to either side. The difference lies in the part of the curve that continues below the springing line. That part contributes little structurally and is mainly a matter of form. The voussoirs are often aligned not on the centre of the arch but on a point lower down, which sets both the outline and the pattern of the joints. The load usually comes down onto columns or piers through capitals or impost blocks that project slightly into the opening.

• Round horseshoe arch: semicircular crown
• Pointed horseshoe arch: crown formed by two arcs
• Decorative types used alongside multifoil arches

In survey and restitution

The key measurements are how far the curve drops below the springing line and the width at the narrowest point. That ratio shows the geometric rule behind the arch. Record the direction of the voussoir joints as well. In restitution work these data are used to recover the original form of a distorted or infilled arch. In 3D, building the arch from a curve through measured points rather than from an ideal circle gives a more accurate result.