This is an interior view of Wells Cathedral - showing one of the three famous 'scissor' arches, which support the weight of the central tower.
The arches were retro-fitted in around 1340 to stabilise a new tower that had been built about 15 years earlier, and was already showing signs of instability and potential collapse.
The arches were designed by the delightfully-named William Joy, and are an elegant solution to a common Medieval construction problem.