Glossary - (Illustrations)
Abutment - solid support absorbing the outward thrust of the arch.
Crown - top part of arch. including the keystone.
Extrados - outer curve of the voussoirs.
Haunch - the lower section of the arch, below the crown, where outward thrust is most pronounced.
Impost - projection marking the point from which the arch springs from its support.
Intrados - inner face of voussoirs. (also soffit).
Keystone - the voussoir at the centre of the arch, often larger and decorated.
Rise - vertical height between springing line and underside of keystone.
Semi-arch - an arch where only one part of its span is complete as for example in a flying buttress. - (Illustration)
Squinch - an arch which spans the angle formed by two walls meeting (Illustration). Usually to carry a dome, a form which evolved into the pendentive.
Span - distance between vertical face of abutments or supports.
Spandrel - the triangular shape contained by the side of an arch, a horizontal line drawn through its crown, and a vertical line on the end of the span.
Springer - horizontal voussoir at start of arch, also known as "skew backs."
Springing line - level at which the arch springs from its support.
Voussoir - the radiating wedge-shaped blocks forming the arch.