Why Are Cathedrals All the Same Shape? A Plan That Had Jobs to Do

The standard arrangement divides into recognisable elements each with a purpose. The nave is the long western arm where a congregation stood, and it was public space used for far more than services. The transepts crossing it produce the cross shape and supply additional altars and processional routes. The crossing beneath them frequently carries a tower. The choir or chancel at the eastern end held the clergy and the daily offices, and was separated from the nave by a screen in most arrangements. The sanctuary at the far east holds the principal altar. An ambulatory may run around behind it, allowing movement past chapels without disturbing services, and radiating chapels open off it.

Each feature answers a practical requirement:

• •Eastward orientation, which is a settled convention with a stated theological basis
• •Separation of clergy from congregation, which the screen enforces physically
• •Processional routes, which determine the widths of aisles and the placement of doors
• •Pilgrim circulation past a shrine, which is why ambulatories and radiating chapels exist
• •Multiple altars for private masses, which multiplied chapels enormously in the later medieval period
• •Burial, since floors, chapels and walls accommodated tombs in quantity

The cross-section of the nave repeats as consistently as the plan and for structural reasons. An arcade of arches at ground level separates nave from aisles and carries the wall above. A middle level, either a gallery over the aisles or a shallow passage, occupies the height corresponding to the aisle roof. A clerestory above that carries the windows lighting the nave, which is the whole point of raising the nave above the aisles in the first place. Above the vault, a timber roof covers everything and is invisible from inside. Every one of those levels is doing structural work, and the proportions between them are the main thing that distinguishes one period and region from another.

The structural problem is the same one every large masonry building faces and the solutions are visible throughout. A stone vault pushes outward as well as down, so that thrust must be carried to the ground, which is done by thickening the walls, by buttressing them externally or by flying buttresses carrying the force across the aisles to piers standing clear of the building. Pinnacles weight those piers to steepen the line of force within the stone. Towers concentrate enormous loads at the crossing, which is where collapses most often occurred and where several cathedrals lost a tower during or after construction. Foundations on poor ground caused settlement and leaning that is visible in many buildings. The whole exterior is legible as a diagram of where the forces go.

Construction times of a century or more are normal and the reasons are financial and practical rather than technical. Funding arrived irregularly from donations, from pilgrim offerings and from bishops' revenues, so work proceeded as money allowed and stopped when it did not. Building began at the east end so that the sanctuary could be used while the rest was unfinished, which is why the eastern parts of many cathedrals are in an earlier style than the western. Fires, collapses and changes of plan interrupted repeatedly. Styles changed during construction, which is why a single building frequently contains several, and reading those changes along the length of a nave is a standard exercise. Several were never finished at all.

Nave, transepts, choir and sanctuary each answer a requirement, with the screen separating clergy from congregation and the ambulatory moving pilgrims past a shrine without disturbing services. The nave is raised above the aisles to carry windows above the aisle roofs. Building began at the east so it could be used while unfinished, which is why western parts are usually later in style.