Definitive Trade Reference

Arch Terminology & Visual Anatomy Glossary

Every component of a traditional compressive arch serves a specific structural purpose. Explore definitive self-contained definitions, interactive vector inspector diagrams, and jobsite usage.

Visual Interactive Glossary

Anatomy of an Arch — Terminology & Geometry

Click or hover over any term or interact with diagram hotspots to inspect key architectural components.

Interactive Anatomy Model Click a part to inspect

Keystone / Apex Structural Crown

The central wedge-shaped stone or framing gusset at the crown locking the arch into compression.

Keystone / Apex Structural Crown

The central wedge-shaped stone or framing gusset at the crown locking the arch into compression.

💡 Must sit precisely on the centerline. In masonry, an odd count of bricks ensures a solid keystone at the apex.

Intrados (Soffit) Inner Curvature

The inner, bottom concave surface of the arch opening determining clear walk-through height.

💡 Arc length of the intrados dictates the exact cut length of flexible drywall, trim casing, or formwork skin.

Extrados (Back) Outer Boundary

The upper, exterior convex curved boundary of the arch ring or framing plate.

💡 The area between the extrados and horizontal ceiling header forms the spandrel wall framing.

Voussoirs & Wedges Masonry Blocks

The wedge-shaped masonry units or segmented framing blocks forming the curved arch ring.

💡 Radial mortar joints between voussoirs taper from 1/8" at the intrados to 3/8" at the extrados.

Springline & Springers Baseline Elevation

The imaginary horizontal baseline from which the curve begins springing away from vertical jambs.

💡 All vertical rise (sagitta) measurements are taken strictly perpendicular from this springline.

Skewback & Impost Abutment Bearing

The inclined or horizontal seating surface on piers bearing outward diagonal arch thrust.

💡 For flat jack and shallow segmental arches, skewbacks must angle 60° to 70° toward the radius center.

Comprehensive Glossary Index

12 Fundamental Arch Structural Elements

Self-contained architectural and engineering definitions for featured snippets and professional trade reference.

Keystone

Apex Crown

The central wedge-shaped stone or brick at the highest crown of an arch that locks the entire structural ring in place under compressive force.

Voussoir

Arch Ring

Any of the wedge-shaped tapered stones or bricks that form the curved compressive ring of an arch between the springline and the keystone.

Intrados (Soffit)

Inner Curve

The inner curved underside surface of an arch opening, measured along the ceiling perimeter where casing trim and drywall liners attach.

Extrados (Back)

Outer Curve

The exterior, upper curved boundary line of the arch ring that interfaces with the surrounding wall masonry or framing studs.

Springline

Baseline

The horizontal imaginary line connecting the points where the vertical jamb piers end and the curved arch profile begins.

Impost

Pier Capital

The horizontal projecting stone bracket, capital, or wood block on top of a supporting jamb pier that directly receives the arch thrust.

Skewback

Bearing Seat

The angled, sloping bearing surface on an abutment pier that provides the perpendicular radial seat for the lowest springer voussoir.

Spandrel

Wall Infill

The triangular wall area bounded by the outer curve (extrados) of an arch, a horizontal line through the crown, and a vertical line from the jamb.

Crown (Apex)

Highest Point

The highest central peak of an arch ring where the keystone is seated and compressive thrust lines converge.

Sagitta (Rise)

Height H

The vertical distance measured perpendicularly from the horizontal chord springline to the highest point of the intrados soffit.

Haunch

Mid-Shoulder

The middle portion of the arch ring located between the crown apex and the springline base (typically around one-third of the rise height).

Abutment

Thrust Wall

The solid masonry wall, double-king stud jamb, or concrete foundation that resists both the downward gravity load and outward lateral kick of an arch.

Explore Layout Guides & Arch Calculators

Types Comparison How to Draw Arches Brick Voussoirs Semicircular Arch Segmental Arch Elliptical Arch

Expert Knowledge Base

Frequently Asked Questions

Common carpentry, masonry, and geometric questions regarding arch calculations and construction layout.

Q1 What is the difference between the intrados and the extrados of an arch?

The intrados (also called the soffit) is the interior curved lower surface of an arch opening. The extrados is the outer, upper curved perimeter surface of the arch ring. Because the extrados has a larger radius than the intrados, its perimeter arc length is always longer.

Q2 What is the purpose of the keystone in a masonry arch?

The keystone is the central wedge-shaped voussoir stone or brick positioned at the highest apex point (the crown) of an arch. Dropped into place last, the keystone locks all surrounding voussoirs together into compressive ring equilibrium.

Q3 What is the springline (or springing point)?

The springline is the horizontal baseline where the vertical support wall or jamb ends and the curved arch begins. The points at either side where the curve departs from the vertical support are called the springing points.

Q4 What is an impost and a skewback?

An impost is the horizontal projecting block, moulding, or pier capital directly beneath the springing point that supports the arch. A skewback is the angled, sloping bearing surface on an impost or abutment that provides the radial seat for the lowest springer voussoir.

Q5 What is a spandrel in architecture?

A spandrel is the triangular wall area bounded by the outer curve (extrados) of an arch, the horizontal line passing through the crown, and the vertical line rising from the springline jamb.