Four ribs meeting at the centre of a bay and nothing else: quadripartite rib vault. Six ribs, with one extra crossing the bay from side to side: sexpartite. Extra ribs running from the springing to the ridge but landing nowhere structural: tiercerons. Short ribs that connect one rib to another and touch neither the springing point nor the apex: liernes. And if every rib springs from a single point in an even spread of identical curves, like the underside of an opened umbrella, you are standing under a fan vault and you are almost certainly in England.
That is the whole identification problem solved in a paragraph. The rest of this is why those differences exist, because once you know what each change bought the mason, you stop memorising the names and start reading the ceiling as what it is: a diagram of a structural argument, drawn at full size in stone, by people who had no calculations and one chance.
You want a wide nave, a tall one, and a stone ceiling over it because stone does not burn and your timber-roofed predecessor did. Stone has no useful tensile strength, so the only way to span with it is in compression, in an arch. Extend an arch along a length and you have a barrel vault — strong, but it pushes outward along its entire length, which means continuous thick walls and almost no windows.
Cross two barrel vaults at right angles and the outward thrust stops being continuous. It collects at four corners. That is the whole trick, and everything that follows is a refinement of it. Thrust gathered at points means the wall between those points is doing nothing structural. It can become glass. The Gothic cathedral is not a wall with holes in it; it is a frame of concentrated loads with infill between, and the ceiling is where you can see that logic most clearly, because nobody plastered over it.
No Ribs, No Mercy
The intersection of two barrel vaults produces a sharp edge — a groin — running diagonally from corner to corner. No moulding, no projection, just a crisp arris where two curved surfaces meet. If you look up and see four curved panels meeting along clean diagonal creases with nothing standing proud of them, that is a groin vault.
Here is why nobody built them very large for very long. A groin vault is a single continuous surface, so it has to be built on a single continuous formwork — a complete timber false ceiling, filling the entire bay, carpentered to the exact double-curved geometry of the finished vault. That is an enormous amount of timber and an enormous amount of skilled joinery for a thing you then dismantle. Worse, the geometry only behaves if the bay is square. Make it oblong and the diagonal groin runs at a different radius from the cross arches, and the whole surface starts warping in ways that are miserable to set out and worse to build.
A note on Durham, since half the internet has it wrong: the high vaults at Durham are rib vaults, not groin vaults, and they are a standard exhibit in the argument about when rib vaulting began. If an article tells you Durham is your groin vault example, that article has not been inside it.
The Rib Changes the Build, Not Just the Look
A rib vault puts a permanent stone arch along each of those diagonal lines first, and fills the pockets between afterwards. Visually the difference is obvious: the diagonals now project downward as moulded stone members, catching light on one side and throwing shadow on the other.
Structurally — or at least procedurally — the change is bigger than it looks. You only need timber centering under the ribs themselves: narrow, cheap, reusable arch formers rather than a whole false ceiling. Build the ribs, let them set, strike the centering, move it to the next bay, and infill the webs off the ribs you have just made. The vault becomes a repeatable operation rather than a one-off event, and the timber goes down the nave with the masons instead of being consumed bay by bay. In practice that is the single biggest reason the rib wins: it turns an expensive bespoke build into a production line.
It also solves the oblong bay. Because each rib is an independent arch, you can adjust the rise of each one separately so they all reach the same ridge height despite spanning different distances. With a pointed arch you can do this almost arbitrarily — change the angle, keep the height. That is what the pointed arch is actually for. Not pathos. Geometry management.
The quire at Canterbury is where most visitors in England first meet the new French manner; the monk Gervase of Canterbury wrote the eyewitness account of the fire and the rebuilding that every historian since has worked from, including the arrival of the mason William of Sens.
Count to Four, Then Count to Six
Stand in the middle of one bay and count the ribs converging overhead. Two diagonals crossing at the centre, with transverse arches dividing bay from bay: four compartments of webbing, quadripartite.
Now look for an extra rib crossing the bay side to side, from the middle of one wall to the middle of the other, splitting each half of the bay again. Six compartments, sexpartite. It usually pairs with an alternating rhythm in the piers below — a major pier, then a minor one — because the extra rib needs somewhere to land. If the columns down the nave alternate fat and thin for no apparent reason, look up. The answer is on the ceiling.
The Rib That Goes Somewhere New
A tierceron is an extra rib that springs from the same point as the diagonals but runs to a point on the ridge rib rather than to the opposite corner. That is the entire definition, and it is the one the search results never give you.
So the field test is destination, not count. Follow each rib with your eye from where it leaves the wall. Does it go corner to corner? Diagonal. Does it go wall to wall across the bay? Transverse or sexpartite. Does it leave the springing with the others, then peel off and die into the ridge line somewhere short of the far corner? Tierceron.
What it bought is less about load and more about surface. It breaks the webbing into smaller panels, which are easier to build and less prone to the sagging that large web panels suffer. It also turns the spread of ribs at each springing into a fan of five or seven members instead of three, which reads continuously down the length of the nave as a tunnel of stone branches. Exeter's nave is the one to see — the ridge rib runs the full length and the tiercerons spring in repeating clusters either side of it, and no photograph has ever done it justice.
The Ribs That Connect Nothing to Nothing
A lierne touches neither a springing point nor the ridge. It is a short rib running from one rib to another, used to complete a pattern — stars, lozenges, nets. Once you have that rule, liernes are the easiest thing in the vocabulary to spot: look for any rib whose two ends both land on other ribs.
By this stage the ribs have stopped pretending. They are a decorative net laid over a vault surface, and the bosses at the junctions get carved, painted and gilded because everyone has accepted that this is now a display. Which brings us to the argument.
Was the Rib Ever Structural? Nobody Has Settled It
The standard story — ribs as the permanent skeleton, webs as infill, the whole thing a rational frame — comes largely from Viollet-le-Duc's Dictionnaire raisonné, and it suited the nineteenth century enormously because it made medieval masons into proto-engineers. Pol Abraham's Viollet-le-Duc et le rationalisme médiéval, published in the 1930s, attacked that reading directly, arguing the webs are largely self-supporting shells and the ribs far closer to decoration than the rationalists claimed. The evidence usually cited on his side is that vaults have survived bomb damage with ribs destroyed and webbing standing.
My own position, and it is a practitioner's one rather than a historian's: the rib's clearest and least arguable value is during construction, not after it. It lets you strike and reuse your centering. Whether it is doing much once the mortar has gone off is a separate question from whether it was worth building, and the two keep getting conflated. Treat anyone who tells you it is settled with suspicion.
While you are arguing with yourself, look at the webbing between the ribs. Is it coursed ashlar laid in neat parallel rings, or rubble, or thin brick? Is it level with the ribs or set behind them? That surface tells you more about how the thing was actually built than the rib profile does, and nobody photographs it.
The Cone Wins
A fan vault abandons ribs-as-structure entirely. Each springing generates a half-cone — a conoid — and every rib on that conoid is the same curve, spreading at equal angles. Because all the ribs are identical, any horizontal cut through the conoid is a circle. That is the geometric definition, and it is also the identification test: if the ribs are all the same length and curvature and spread evenly like the spokes of an opened umbrella, with horizontal bands running across them as true arcs, it is a fan vault. If the ribs have different curvatures, it is not, however elaborate the pattern.
The critical thing about how they are made: the ribs and the panel between them are carved on the same jointed stone block. It is not a frame with infill. It is a set of shaped panels, each cut to a precise geometry, assembled like a tiled shell. That is why fan vaults look machined — they effectively are, by templates — and why the form is almost exclusively English and almost exclusively late. Gloucester's cloister is usually credited as the earliest surviving example; King's College Chapel in Cambridge is the one everyone travels for.
Dating a Building by Its Ceiling
As a rough rule of thumb, and treat it as a starting hypothesis rather than a verdict: plain groin vaults read earlier than ribbed ones; simple quadripartite and sexpartite rib vaults belong to the first confident Gothic century; tiercerons mark the moment decoration starts outrunning necessity; liernes and net vaults come later again; and fan vaulting is the last act, English, and very late. Cathedrals were built over centuries and repaired after fires, so you will often walk from one period to another halfway down a nave. That discontinuity is usually the most interesting thing in the building.
Photograph It, Then Sketch It in Thirty Seconds
Phone flat on your back on the floor if the vergers allow it, lens straight up, centred on one bay, and shoot the whole bay rather than a pretty fragment — a cropped vault is an unidentifiable vault. Turn the flash off. Underexpose slightly, because the webs are pale and the ribs are what you need to read.
Then do the thing that actually makes it stick. In your notebook, draw the rectangle of one bay and put in only the ribs, as straight lines in plan. Thirty seconds. No curves, no shading, no bosses. Diagonals corner to corner; a transverse line if it is sexpartite; lines from the corners to the ridge if there are tiercerons; and any line connecting two other lines is a lierne. The pattern that comes out of your pen is the vault's actual classification, stripped of the carving and the gilding that make it hard to see from below. Do that in four buildings and you will never need the field test again.
What you are reading when you look up is not decoration that happens to be structural, or structure that happens to be decorative. It is the visible record of a problem being solved repeatedly over four hundred years by people who could not calculate the answer and so built it, watched it, and adjusted. The ribs get more numerous as the masons get more confident and the geometry gets more controlled — right up to the fan vault, where the whole thing becomes a single disciplined shape and the argument ends.
Go and stand under one. Count the ribs. Follow where they go.
