information resourcesChildren and television violencelink to document abstractslink to short briefings documents link to news zone Click for an introduction to cathedrals and stained glass in France. news resources at interesting site links at abelard's news and comment zone socialism, sociology, supporting documents described quotations at, with source document where relevant
site map Energy - beyond fossil fuelsLoud music and hearing damageWhat is memory, and intelligence? Incautious claims of IQ genes economics and money zone at - government swindles and how to transfer money on the net technology zone at how to survive and thrive on the web France zone at - another France visit abelard's gallery rssxml

back to abelard's front page

click to return to the France Zone home page

gothic cathedral and church construction

New translation, the Magna Carta

This page helpful? Share it!


new : stone in church and cathedral construction illustrated

fortified churches, mostly in Les Landes

cathedral labyrinths and mazes in France
using metal in gothic cathedral construction

Germans in France
cathedral destruction during the French revolution, subsidiary page to Germans in France

Cathedrals in France

on first arriving in France - driving
France is not England
paying at the péage (toll station)

Click for motorways and motorway aires in France.

Transbordeur bridges in France and the world 2: focus on Portugalete, Chicago, Rochefort-Martrou
Gustave Eiffel’s first work: the Eiffel passerelle, Bordeaux
a fifth bridge coming to Bordeaux: pont Chaban-Delmas, a new vertical lift bridge

France’s western isles: Ile de Ré
France’s western iles: Ile d’Oleron

Ile de France, Paris: in the context of Abelard and of French cathedrals
short biography of Pierre (Peter) Abelard

Marianne - a French national symbol, with French definitive stamps

la Belle Epoque
Grand Palais, Paris

Click to go to pages about Art Deco at

Click to go to 'the highest, longest: the viaduct de Millau'

Pic du Midi - observing stars clearly, A64
Carcassonne, A61: world heritage fortified city

Space City, Toulouse

the French umbrella & Aurillac

50 years old: Citroën DS
the Citroën 2CV: a French motoring icon

the forest as seen by Francois Mauriac, and today
Les Landes, places and playtime
roundabout art of Les Landes

Hermès scarves

Hèrmes logo

bastide towns
mardi gras! carnival in Basque country
country life in France: the poultry fair

what a hair cut! m & french pop/rock

Tour de France 2017
Le Tour de France: cycling tactics illustrated

church and cathedral construction
building a cathedral
how gothic cathedrals stay up
let the light flood in
so now for a bit on stone
other major materials used in building cathedrals
naming parts of a gothic cathedral - interior
gothic cathedral building start dates, including precursor buildings
recommended reading
end notes

church and cathedral construction

The building of gothic cathedrals developed in step with the development of the design process. Through the medieval apprenticeship system, accumulated knowledge was passed down. Over a period of about a century and a half, designs increased in complexity and sophistication as the new techniques developed into a coherent ‘gothic’ style.

Added to this accumulating knowledge taken from experience, came the use of small-scale models, constructed similarly to the proposed full-scale building, and used to test the overall stability of a design. Detailed testing was made ‘on the job’, by building the cathedral one bay at a time, while using proven elements from previous designs.

So it was that cathedral construction evolved, with ribbed vaults and pointed arches appearing at the start of the eleventh century. This first major innovation in gothic cathedrals - the pointed arch - replaced the rounded Roman arch and enabled the building of more complex and higher buildings. This evolved to include the ribbed, arched vaults of large buildings, which were able to span much large spaces than could the Roman arch.

cross-section of Laon cathedral nave, showing the unusual four-tier construction.
Cross-section of Laon cathedral nave, showing the unusual four-tier construction
[Shaded part corresponds to buttressing illustration below -
click on shading to jump]

By trial and error came the flying buttress to counteract outward pressures, so allowing ever taller and more complex structures. Flying buttresses were first recognised as being used in Notre Dame in Paris shortly before 1180.

Later, throughout the twelfth century, came the technology that gradually removed the requirement for walls to carry loads. The gothic builders discovered that it was no longer necessary to build curtain walls between the load-bearing skeleton of the church, because the skeleton carried all the weight. This allowed the builders to become much more adventurous, and to fill the spaces between the load-bearing pillars and ribs with glass.

The leading person to consolidate all these building techniques was Abbé Suger (Abbot of St. Denis, 1081-1151).

e = window.adsbygoogle || []).push({});

building a cathedral

When building a cathedral, the usual tendency was to get something up and running as quickly as possible. Thus, you might build the chancel first, and then extend outwards as time and money became available.

In some sense, a cathedral is like a building made of playing cards, each section leaning on others in some sort of equilibrium. This also will determine the stages of the construction. It will be expensive to shore up one part while another is being built, only to have to remove it later. Much of the building process of the gothic cathedrals was innovative and experimental. Bits did tend to fall down from time to time.

While one process was to build from the chancel outwards, I do believe some started with building the nave, a series of simple square boxes. But this means starting high up, instead of having the slopes of a chancel to lean against. However, the simpler method of starting from the west could easily be cheaper. If you look carefully at the west (main) entrance, you will see that between each door there is also the base of a massive buttress, disguised by the statuary around the doorways. These buttresses again block the house of cards, in this case preventing the cathedral facade from moving outwards under the pressure of the vaults.

On this postcard of Amiens cathedral, the west facade buttresses have been highlighted in blue. The buttresses are disguised by statues and other carving, and the artistic horizontal storey divisions.

Amiens cathedral - west facade, showing buttresses.
Amiens cathedral - west facade, showing buttresses

For reasons that are not exactly clear, the bays are numbered from the west facade. Thus the bay nearest to the chancel is often called ‘the last bay’. Maybe this is an indication that construction started from the west end of the cathedral.

how gothic cathedrals stay up

Gothic cathedrals have large curtain walls filled in with stained glass, walls that do not support the weight of the vaulting and roofs. The cathedrals are often very tall and have to resist considerable side pressures from wind. In general, the further north, and the taller the cathedral, the greater the wind pressures. When you reach Britain, the cathedrals are generally lower and with lower-pitched roofs [1], in part because of the higher wind speeds.

[based on 19th century drawing of Amiens cathedral]

half cross-section of Amiens cathedral to show buttressing and roof.The main weight of the cathedral structure is carried downwards by the tall pillars that march along the nave and the side aisles. The pointed arch vaulting directs more of the force downwards than does the Roman arch, but there remain lateral (sideways) forces to be managed. Down the marching pillars and bays there are counter-forces from bay to bay, but pressures remain towards the outside, for which solutions had to be found.

There gradually developed an external system of buttressing, which applied counter-acting force sideways towards the cathedral wall. This resists the tendency of the walls to bulge out from the lateral pressures.

There are no lateral forces exerted by the roof in situ, for the roof is framed in wood and cross-pinned to hold it together; but, of course, the wind forces do transmit force laterally. The upper flying buttress redirects the wind forces from the roof and the clerestory wall, guiding them downwards into the pier buttress.

The lower flying buttress performs the same duty, but for the outward lateral forces being exerted by the nave vaulting.

The pier buttress, as you can see from the diagram, blocks the equivalent force from the vaulting of the side aisle. The pier buttress also supports the flying buttresses, bracing them so the cathedral wall does not move outwards. The pier buttress also transforms the still sideways forces into downward ones.

The pinnacle adds further weight to the pier buttress, helping to anchor it against sideways pressure. Piers are optional and positioned according to the stresses found by the builders. Remember that these arrangements were worked out as the builders noticed problems, modifying the structure if they noticed.

Flying buttresses, close-up
Flying buttresses [at Laon cathedral, corresponds to shaded area in first diagram]

Note the buttresses marching down the side of the cathedral. Between each pair of buttresses in this innovatory construction technique, there is no serious weight coming down from the roof and the vaults. These in-between walls now have to carry relatively very little weight and, therefore, can be opened up to accommodate the glory of large stained-glass windows, thus letting in the light. Note the arch construction of the flying buttresses, see also wheels within wheels.

let the light flood in

3-D diagram to indicate position of different vaultinggothic vaulting

Roman arches are fine for spherical domes and barrel roofs, but not for more complex shapes.

In the diagram to the left, note that the arches for an oblong bay must span three different distances. This can be handled effectively by the use of pointed arches, by varying the steepness of the arcs, thus terminating all three arch lengths at the same height above the nave. This is shown by the colour coding in the diagram.

The gaps between the arches can then be filled by simple curves.

In some cathedrals, a sexpartite design is substituted in order to further spread the vault load. For example, an extra arch would run between the two mauve arches, joining with the tip of the yellow arches. Each end of this extra arch will then be supported on two more intermediate pillars, normally of a narrower section, which will thus transmit some the of the weight downwards. Bourges is an example of sexpartite vaulting, the following illustration has been shaded to show one sexpartite section of the vaulting:

Bourges sexpartite roof vaulting. Credit: AEngineer
Bourges sexpartite roof vaulting. Credit: AEngineer

Further notes on the vaults and the stone out of which they are made.

Left: Labelling the parts of the nave
Labelling the parts of the naveThis illustration should be studied in concert with the cross-section of Amiens cathedral, above.

Note how the clerestory stained-glass windows fit between the ribs and up into the associated vaulting. The vaulting is supported by the ribs [see diagram above]. The ribs descend onto the pillars, which reach down to the ground.

Outside the cathedral, the flying buttresses press against the cathedral wall in line with the pillars on the inside, leaving the window walls unobstructed.

Abbé Suger, who was the effective inventor of this style at the royal church of Saint Denis, had a poetic and imaginative approach to this new light which he had invited into the church. He intended that the beauty that he brought into the church would raise the worshipper from the material to the non-material, bringing him closer to god.

Abbé Suger [1081 - 1151] had inscribed above the doorway of Saint Denis,

“Whoever thou art, if thou seekest to extol the glory of these doors,
Marvel not at the gold and the expense, but at the craftsmanship of the work,
Bright is the noble work; but being nobly bright, the work
Should brighten the minds, so that they may travel, through the true lights,
To the True Light where Christ is the true door ...
The dull mind rises to truth through that which is material.”

The symbolism of Saint Denis and other cathedrals was often abstruse and esoteric. Thereby, Suger, in the manner of great teachers, sent complex messages to the extremely highly educated classes, while attempting to communicate with all people, by what he termed “analogical vision”, through the jewelled beauty and artistry of his visionary construction. For some further details on the construction of levels or storeys.

so now for a bit on stone

Well, I suppose we all know what stone is, it’s that stuff that’s all over the place on this fascinating planet. Well, that’s mostly what I know about it anyway; but I hear tell that there are people who spend their whole lives studying stuff like stones, so I imagine there must be a lot more to it than that. (Maybe eventually, I will find an enthusiastic expert who can write some plain English on the subject for me.)

Just one or two notes relevant to cathedrals. The stone needs to be hard enough to stand up, and to last a thousand or two years, and easy enough for a medieval craftsman to work with rather primitive tools.

Stone is very strong in compression, and only about a tenth of that strength in bending and sheering (tension, stretching), while the medieval mortar is about a tenth of that sheering/bending strength.

compression forces on Gothic and Roman arches.

Notice the greyed stones in the arches of diagrams A and B. On the arches, you will see a small triangular diagram. If nothing were holding the shaded stone place, it would fall to the earth; but locked in place, weight (force) is coming down on the stone (the stone, of course, also has some weight). That force is directed mainly along the stone.

As you will see, the stone in the pointed arch [A] is at a more vertical angle than the stone in the Roman (rounded) arch [B]. This means that a greater proportion of the load on the stone is transmitted downward and less outward than is the case with the rounded arch. The differing forces are represented by the differing lengths of the sides of the triangle, and these can be calculated. Be aware that in general the wider the span, the greater the forces.

Those nice vaults under which you are standing can be estimated to weigh towards 300,000 kilograms for the quadripartite vault, and towards 400,000 kilograms for the sexpartite [2] - a nice headache for you if the medieval craftsmen were slacking on the job. Note that the sexpartite tends to cover the equivalent of two quadripartite vaults, thus making the overall weight of sexpartite vaults less and thereby reducing the stresses throughout the structure. (Bourges can be thought of as a very special, high-tech cathedral.)

Moving to diagram C above, you will note that, at the overhang there are forces inclining the stone to bend downwards. Tensile forces are generated at x and compressive forces are generated at y, and thus the pressure is to pull the stone apart at x and tear it apart. In the arch stones, similar forces are being applied. Note that, were the stone at C to be made of butter, it may tend to sheer rather than bend.

And that’s enough about stone for here.

other major materials used in building cathedrals


Until very recently, the roofs of the great cathedrals were framed in massive and complex structures of wood, often known as ‘the forest’.

the forest supporting Soissons cathedral roof
the forest supporting Soissons cathedral roof
credit: Ministère de la Culture (France)

Due to its vulnerability to fire, the‘forest’ has been a great bane for gothic cathedrals from the earliest times. This regularly caused great damage to the cathedral at large, by damaging the stone work, as well as setting fire to the cathedral structure and furnishings.

Now, restoration is turning to reinforced concrete for roof framing as, for instance, at Noyon. Iron and steel has also been used on occasion in the past (see Chartres - roof space: le charpente de fer).


In the days when these cathedrals were built, iron was a very expensive material, probably imported from Spain. A cathedral has some relationship to a playing card construction, with various parts of the structure leaning against other parts.

Sainte-Chapelle, near the Notre Dame cathedral in Paris, a fairly small structure, was built over a period of only five to ten years. It was put up by Louis IX, in a rush, to house the supposed relic, the Crown of Thorns,and so Louis IX could race off on the seventh crusade. Amazingly, Louis had paid more than three times as much for the Crown of Thorns than it cost to build Sainte-Chapelle. Note, a reliquary is the decorated box that holds a sacred relic. The design of reliquaries has been influenced by church design, and visa versa. The shrine of Edward the Confessor in Westminster Abbey is sometimes also regarded as having a visual reference to Sainte-Chapelle.

A reliquary, used to house a sacred relic; from Laon cathedral Sainte-Chapelle, drawing by Decloux 1857 Shrine of Edward the Confessor, Westminster Abbey
A reliquary, used to house a sacred relic; from Laon cathedral Sainte-Chapelle, 1857 Shrine of Edward the Confessor, Westminster Abbey. Image credit

Interestingly, the chapel incorporated a form of iron reinforcement, with two ‘chains’ of hooked bars encircling the upper chapel, the main part of the structure. Further, there were iron stabilisers across the nave (with a vertical tension bar).

Drawing by Lassus of reinforcing bars
Drawing by Lassus of reinforcing bars

Also, an impressive eight-pointed iron star helped hold the apse together. Its iron bars radiated from a central collar. (The drawings above and below were made by Jean-Baptiste-Antoine Lassus during the restoration of Sainte-Chapelle.)

Drawing of eight-ponted iron star by Lassus
Drawing of eight-ponted iron star by Lassus

For further illustrations, see chaînage, or stabilising rods.
See also using metal in gothic cathedral construction.

Because of the rather dodgy stability of the gothic buildings, later additions of iron stabilisation can be seen in many cathedrals, for example in Westminster Abbey.

Iron stabilising bars in Westminster Abbey [indicated with blue arrows]
Iron stabilising bars in Westminster Abbey [indicated with blue arrows]

Similarly, wooden shoring is not uncommon during the recent century, while in Sainte Chapelle, this innovatory reinforcement is hidden from sight, incorporated into the building.

Sainte Chapelle suffered much during the centuries, from repeated fires [1630 and 1736] and even flooding of the Seine [1690]. And then, of course, came the French Revolution, Sainte Chapelle was used as a flour store, a club room and a judicial archive. Sainte Chapelle was not handed back to its intended use for about forty years. After the depredations of the Revolution, excellent restoration and refurbishment was supervised by Jean-Baptiste-Antoine Lassus [1807-1857], and continued by Eugène Emmanuel Viollet Le-Duc [1814 – 1879].

naming parts of a gothic cathedral - interior

plan of a gothic cathedral - interior
[1st chapel: Chapels in the Ambulatory are traditionally numbered from the South,
while main bays are numbered from the West.]
sometimes the side aisles are doubled Also named the choir, from its French name, choeur vestibule, found in earlier churches

Christianist cathedrals and churches are usually built to have a floor-plan shaped like a cross. The head of the cross is generally oriented towards the East, towards Jerusalem. Thus the transept, running north and south, is the cross-bar of this cross. The foot of the cross, at the west end, accommodates the spectacular portals of the main entrance . (Some cathedrals have five aisles - for example, Bourges.)

Transept: Going across the main body of the cathedral, with north and south arms, sometimes with side doors.
Narthex: A vestibule, found in some earlier churches.
Side aisles: these can sometimes be double.
Nave: the space between four pillars is called a bay or a transverse section. The vaulting went diagonally across between the piers of each bay. [Nave is the French word for ship or vessel]
Chancel: is the part of the church to eat of the transept. The word choir is sometimes used sloppily for this area, but more sensibly just applied to the area where the choir sings. The chancel includes the high altar at its eastern end.
Apse: semi-circular or polygonal ending to the chancel.
Ambulatory: walking area around the chancel.
Chevet: a name used in France for the eastern end of the church in general, including the chancel, apsidal chapels and ambulatory.
Rood screen (choir screen, chancel screen, or jube) : an ornate stone or wood partiton dividing the nave from the choir.

The five-arch portal at the west facade of Bourges cathedral.
The five-arch portal at the west facade of Bourges cathedral

gothic cathedral building start dates,
including precursor buildings

Gothic cathedrals were built over extended periods, often centuries. Frequently, the work was started then stopped for years or even decades, according to the availability of will and resources. Therefore, the dates below must be read with caution.

Click for gothic cathedral and church construction in France.

For more on cathedrals in France, visit Cathedrals and stained glass in France, where you can find a comprehensive listing of pages on French cathedrals; on stained glass and other related topics.

See also


  • clerestory. Also clerstory, clarestory, clerestorey, clarester, cleer story, clear story, clearstory.
    [Commonly believed to be from the French, clere, clear + story, stage of a building or ‘floor’ of a house. Clere must here have meant ‘light, lighted,’ since the sense ‘free, unobstructed’ did not yet exist. This assumed derivation is strengthened by the parallel blind-story, although this may have been a later formulation in imitation of clere-story. The great difficulty is the non-appearance of story in the sense required before c. 1600, and the absence of all trace of it in any sense in 14th, 15th, and chief part of 16th c. At the same time there is a solitary instance of storys in the civic Rolls of Gloucester [R. Glouc.] (1724), which may mean ‘elevated structure’ or ‘fortified place’. The noun estorie in Old French [OF]. had no such sense, but the past participle estoré meant ‘built, constructed, founded, established, instituted, fortified, furnished, fitted out’, whence a noun with the sense ‘erection, fortification’ might perhaps arise.]

    The upper part of the nave, choir, and transepts of a cathedral or other large church, lying above the triforium (or, if there is no triforium, immediately over the arches of the nave, etc.), and containing a series of windows, clear of the roofs of the aisles, admitting light to the central parts of the building.

    Clearstory is now more common in the USA, whereas European usage prefers clerestory. It is interesting to note that large numbers of the more useful monographs on the architecture of medieval France originate in the USA, while many of the more philosophical studies originate in the United Kingdom.

  • The word triforum has no reliable etymology, but has come to mean the arcaded gallery below the clerestory windows and above the nave aisles and side bays. More effectively, I would use triforium as describing a minor gallery somewhere above the nave aisle level. This word was first used by Gervase of Canterbury, c. 1185, to refer to a gallery at Canterbury Cathedral, and was used only in the context of Canterbury Cathedral until about 1800. [See also end note 3.]
    There is a much expanded section in cathedrals - an illustrated glossary.

  • Tympanum (in French, tympan) : the half-moon shaped space above the exterior doors of a cathedral, shaded by the archivaults above. The tympanum often illustrates events in the life of Christians, or events in the Bible or in the life of a saint.

A greatly expanded and illustrated glossary can be found at
Cathedrals - an illustrated glossary.


recommended reading

Experiments in gothic structure by Robert Mark Experiments in gothic structure by Robert Mark
MIT Press Five GoldenYak (tm) award

pbk 0262630958
reprint: 1984 /

If you want to understand the structure of the great gothic cathedrals, this is the place to go. Some of it gets a bit technical, Mark used polarised light, epoxy plastic models and wind tunnels to work out the the loadings and stresses in some of the great cathedrals. An absolutely fascinating book to read, if you can stand the hard work and the usual technical manual disorganisation.

As with Painton Cowen, I can not resist giving this book five GoldenYaks, if only because I know of nothing better.

Marker at

Some reference keywords/tags:
church building,eglise,cathederal,gothic cathedral construction,Cathedrale,church constructing,ribbed vault,pointed lancet arch, lancet window, transept,nave,apse,triforium,clerestory,tribune gallery,side aisle, yoked lobed medallion,photos,image,images,photograph,picture,pics,France,Gothic architecture,German bombing,son et lumiere,stained glass windows,Chartres,Rouen,Poitiers,Dax,northern,western,eastern,southern,diagram,diag,illustration,

end notes

  1. The French cathedrals were, in general, serving towns and were a matter of civic pride; there was even the equivalent of a race to build bigger and taller [see Beauvais], just as with modern skyscrapers. In Britain, however, most of the important cathedrals were primarily associated with monastic community buildings, hence the usual cloisters. For the most ‘French’ cathedral in Britain, visit Westminster Abbey - and don’t forget to look up at the vaulting, it is more decorative and elaborate than any I know in France.

  2. Being more precise, and quoting from Mark, “The Bourges sexpartite vault is estimated to weigh 370,000 kg (820,000 lb), that is approximately 400 imperial tons. Whereas, Mark estimates that the Cologne quadrapartites to weigh 270,000 kg (600,000 lb), that is going on 300 imperial tons. [Abstracted from Mark, primarily from p.115.]
    [An imperial (long) ton is 2, 240 lb or 1016.05 kg.
    A short or American ton is 2,000 lb or 907.18 kg.
    Increasingly used is the metric tonne of 1000 kg, or 2, 204.6 lb.]

    Comparing one Bourges sexpartite with two quadrapartites, a Bourges sexpartite covers an area of about 92.2% of two Cologne quadrapartites. Thus, 400 tons at Bourges matches to about (300 tons x 2 x 92.2%), that is 553 tons. You can see from the figures that the sexpartite formation is more efficient in supporting and spreading the load than the quadrapartite configuration [Mark, p.117, note 15].

    The vaults also have rubble, or fill, to increase pressure and to help stabilise them. This included in the weights quoted. Mark estimates that the horizontal outward thrust on the main piers as 28,100 kg (62,000 lb) at Bourges, and 31,300 kg (69,000 lb) at Cologne. The secondary, lighter pier at Bourges, Mark estimates at 11,300 kg (25,000 lb).

  3. Just to be awkward, in parts of Noyon Cathedral, the triforium is below the tribune level.
    Also see the Glossary.

  4. A drawing from Histoire archéologique, descriptive et graphique de la Sainte-Chapelle du Palais by Alfred Pierre Hubert Decloux and Doury, Paris: Félix Malteste, 1857.

  5. Image credit: Mark R. Collins

This page helpful? Share it!




advertising disclaimer


marker cathedrals – introduction: reading stained glass
marker gothic cathedral and church construction
marker cathedrals, an illustrated glossary
marker Chartres - wonder of the world
marker lantern towers of Normandy and elsewhere
marker history of ugly stained glass: Auch, Bazas, Dreux
marker Auch cathedral choir and stalls
marker Rouen and Monet
marker at France pages Dax and church iconography marker photographs, Dax
marker Bazas - iconography and architectural styles
marker Poitiers, neglected masterpiece marker photographs, Poitiers / photos 2
marker Angers, heart of the Angevin Empire marker photographs, Angers
marker Laon, the midst of the gothic transition, with added oxen marker photographs, Laon
marker Saint-Jean-Baptiste de Lyon
marker Notre Dame of Lausanne
marker Senlis - how a typical cathedral changes through the ages
marker Saint-Bertrand-de-Comminges - the cathedral of the Pyrenees

marker Le Mans and Bourges cathedrals - medieval space technology
marker Lausanne rose window - photo-analysis
marker cathedrals in Lorraine - the Three Bishoprics
marker cathedral giants - Amiens and Beauvais
marker Clermont-Ferrand and Agde - from volcanoes to cathedrals

marker Germans in France - Arras cathedral
marker Germans in France - Reims cathedral
marker Germans in France - St. Quentin cathedral
marker Germans in France - Noyon cathedral
marker Germans in France - Cambrai cathedral
marker Germans in France - Soissons cathedral

marker cathedral plans, and facts
marker stone in church and cathedral construction
marker using metal in gothic cathedral construction
marker cathedral labyrinths and mazes in France
marker cathedrals and cloisters of France by Elise Whitlock Rose
abstracts briefings information headlines   loud music & hearing damage children & television violence what is memory, and intelligence? about abelard


© abelard, 2006, 8 february

all rights reserved

the address for this document is

4050 words
prints as 11 A4 pages (on my printer and set-up)

Shaded part corresponds to buttressing illustration, below. triforium