A great medieval cathedral was rarely finished within a single lifetime; construction at Cologne, begun in 1248, was not completed until 1880, and many cathedrals were built, damaged, rebuilt and extended across several centuries by generations of masons who never saw the design realized. The work was organized around lodges of skilled masons, carpenters and glaziers, trades that guarded their geometric and structural knowledge closely and passed it down through apprenticeship rather than written manuals.
Two structural innovations of the Gothic period made the largest cathedrals possible: the ribbed vault, which concentrated a roof's weight onto a skeleton of stone ribs rather than a continuous shell, and the flying buttress, which carried the resulting outward thrust across open space to external piers, freeing the walls between them to be thinned and glazed.
Chartres and Reims cathedrals show the results at their most complete, their naves rising far higher than Romanesque predecessors and their walls largely dissolved into stained glass illustrating scripture for a largely illiterate public. Building on this scale required sustained wealth, usually a combination of episcopal revenue, royal patronage and the offerings of pilgrims drawn by relics housed inside, and a stalled or bankrupt building campaign could leave a cathedral visibly unfinished for generations, its bare stone towers a permanent record of interrupted ambition.