Look above almost any arch, on a cathedral portal, a railway viaduct, a Georgian doorway, and you will find a family of features with precise names and jobs: the spandrel filling the corner, the keystone locking the crown, the hood mould shedding rain, the tympanum carrying sculpture, the archivolts framing it all, and, higher still, the clerestory windows letting in light. These elements are easy to confuse and enormously satisfying to identify. This guide explains each one, what it does structurally or decoratively, and how to tell them apart at a glance.
First, a Quick Refresher on the Arch Itself
An arch is a curved structure that spans an opening by converting the load above it into compression, forces that squeeze rather than bend. Its wedge-shaped stones are voussoirs; the outer curve is the extrados and the inner curve the intrados. The arch springs from imposts at each side and rises to its crown at the top. Everything in this article sits on, above, or around that curve. Because the arch pushes outward as well as down, whatever surrounds it must absorb that thrust, which is exactly why so much interesting architecture accumulates above arches.
The Keystone: The Lock at the Crown
The keystone is the central voussoir at the very top of the arch. Structurally it is the final piece that lets the arch stand on its own, once it is inserted, the temporary timber centering can be removed. Contrary to popular belief, the keystone is not doing more work than its neighbors; every voussoir shares the compression. But its symbolic role as the "completing" stone made it a natural site for emphasis, so masons enlarged it, carved it with faces, scrolls, or dates, and let it project from the wall.
- Spotting it: dead center at the top of the arch, often oversized, projecting, or carved.
- Classic variants: sculpted masks on Baroque palaces, exaggerated keystones on Mannerist gateways, plain elegant wedges on Georgian townhouses. The theatrical versions are a hallmark of the Baroque style.
Spandrels: The Triangles Between and Above
A spandrel is the roughly triangular surface between the curve of an arch and the rectangular frame around it, or between two adjacent arches in an arcade. Spandrels exist because arches are curved and buildings are mostly rectilinear; something has to fill the leftover corners.
That "leftover" space became one of architecture's great decorative opportunities:
- Roman triumphal arches placed winged Victories in their spandrels.
- Medieval and Renaissance arcades filled them with roundels, tracery, or heraldry.
- In steel-framed buildings, the term migrated: spandrel panels are the sections of wall or glazing between the top of one window and the sill of the window above.
Spotting it: find the arch's curve, then look at the corners just above and to each side, the flat wall zone bounded by the curve is the spandrel.
The Tympanum: The Semicircle of Sculpture
When an arch frames a doorway, the door itself is rectangular, and its flat top (the lintel) leaves a half-moon of wall beneath the arch. That surface is the tympanum, and in medieval churches it became the most important sculptural billboard in architecture. Romanesque and Gothic portals filled tympanums with scenes, Christ in Majesty and the Last Judgment being especially common, positioned so every person entering passed beneath the message.
Spotting it: the enclosed panel directly over a door, sitting on the lintel, bounded above by the arch. If it carries carved figures, you are almost certainly looking at a church portal in the Romanesque or Gothic tradition, or a revival of one.
Archivolts: The Nested Frames
Archivolts are the concentric bands of moulding that follow the curve of an arch, effectively the arch's decorated face. Great medieval portals rarely have a single arch; they have four, five, or more nested arches stepping back toward the door, and each band is an archivolt. Gothic sculptors populated them with rings of angels, elders, and saints radiating around the tympanum, so the whole portal reads as a carved halo.
Spotting it: count the concentric rings of moulding or sculpture wrapping the arch. Each ring is one archivolt; the vertical supports below them, often carrying column figures, are the jambs.
Hood Moulds and Labels: The Rain Deflectors
A hood mould (or drip mould) is a projecting moulding that traces over the top of an arch or window on the outside of a building. Its original job is weathering: it throws rainwater clear of the opening so the wall below stays dry. When the same feature is squared off, running horizontally before turning down at each end, it is called a label, and the decorative blocks where it terminates, often carved heads, are label stops.
Spotting it: a slim raised band hugging the outer edge of an arch or window head, usually ending in small carved stops. Ubiquitous on Gothic churches, Tudor houses, and Victorian Gothic Revival buildings of every kind.
The Clerestory: Windows Above the Arcade
Step inside a large church and look up past the arches of the nave arcade. Above them may run a middle story (the triforium), and above that a row of windows near the ceiling: the clerestory (pronounced "clear-story"). It is not attached to a single arch; it is a whole zone of wall, made possible by arches below carrying the loads, whose job is to pour daylight into the tall central space. The concept long predates Christianity, Egyptian temple halls and Roman basilicas used clerestory lighting, and it remains standard in factories, gyms, and modern houses seeking privacy-preserving daylight. The evolution from small Romanesque clerestory openings to the vast glass walls of late Gothic churches is one of the central stories of medieval engineering, told throughout our coverage of Gothic chapter houses and cathedral design.
Spotting Guide: Telling Them Apart
| Feature | Where it sits | Job | Instant identifier |
|---|---|---|---|
| Keystone | Center of the arch's crown | Completes the arch; decorative emphasis | Single oversized or carved wedge at top center |
| Spandrel | Corners between curve and frame, or between two arches | Fills the leftover surface; decoration | Roughly triangular flat zone beside the curve |
| Tympanum | Between door lintel and arch | Sculptural display panel | Half-moon panel directly over a door |
| Archivolt | Following the arch's curve | Decorated face of the arch | Concentric carved or moulded rings |
| Hood mould / label | Projecting over the arch outside | Deflects rainwater; outlines the opening | Slim raised band ending in carved stops |
| Clerestory | High wall zone above an arcade | Daylighting the interior | Row of windows near the roofline |
Putting It Together on a Real Building
Try this sequence on the next arched doorway or church you pass:
- Trace the arch and find the keystone at its crown.
- Slide your eye to the corners: those flat triangles are the spandrels.
- If there is a door with a carved panel above it, name the tympanum, then count the archivolts ringing it.
- Outside, check for a hood mould and its label stops.
- Inside a church, look up for the clerestory above the arcade.
Five minutes of this and the vocabulary sticks for life. If you want instant help, upload a photo to our AI architecture analyzer, it will label these features on your own image and identify the style they belong to. Once you can name what happens above an arch, you can read the ambitions of the people who built it: keeping water out, holding stone up, letting light in, and telling stories overhead.