This is the key stone in the central roof boss representing the Lamb of God carrying the flag, a symbol of the Resurrection. It is the work of William of Sens and is traditionally the point from which he fell from the wooden scaffolding in 1178. Bosses of this type are apparently commonplace in France but very rare in England.
What to see:
the roof boss can be seen high above the crossing of the eastern transepts (Image 1)