A large medieval cathedral commonly spans several centuries between the first stone and the last. The delays were financial and technical, and both are still readable in the fabric.
Funding arrived in instalments
Construction was paid for by a mixture of diocesan revenue, donations, bequests and offerings, none of which delivered a predictable annual sum.
Work therefore proceeded whenever money was available and stopped when it ran out, sometimes for a generation at a time.
Because building resumed under different circumstances, the completed structure records the fortunes of the institution that paid for it as much as any architectural intention.
The east end was built first for a reason
The choir and sanctuary were prioritised because they were the part required for the liturgy, and the building could be brought into use once they were roofed.
A temporary wall closed the western end, and the nave was extended westward over the following decades as funds allowed.
This is why the two ends of a long church often differ in style, and why the change frequently occurs at a clear vertical line in the masonry.
Structural knowledge advanced during construction
Gothic building pushed masonry toward its limits, and the techniques were developed empirically through observation of what stood and what did not.
Failures happened, and collapses of towers and vaults required rebuilding, sometimes to a revised design that reflected what had been learned.
Flying buttresses in particular were often added or strengthened after construction, when movement in the walls showed that additional support was needed.
Interruptions were external as often as internal
War, epidemic disease and shifts in political control removed both the labour force and the revenue, sometimes simultaneously and for long periods.
A master mason's death could also stall a project, since the design existed substantially in his knowledge rather than in a complete set of drawings.
His successor would resume in the manner of his own training, which is another route by which a single building acquires several styles.
Many were never actually finished
A large number of great churches were left incomplete, most visibly with a single tower where two were planned, or a west front never fully built up.
Nineteenth-century restoration completed some of them to designs of that period, which is why certain spires are far younger than the building beneath.
Reading a cathedral therefore means reading a sequence rather than a design, and the joins are usually visible to anyone who looks along the wall.