Actually, there’s nothing particularly special about the keystone, except that it happens to be the last stone to be put in place.

All of the stones in an arch are very important as you can see in this illustration.

Remove any one of them and the arch will collapse.

Why is an arch so strong? Because the weight that the arch is supporting pushes all of the stones towards the central point, squeezing them more and more together.

Now you can see that the keystone is pressed more strongly towards the centre than any other stone, so it has to be stronger than any of the others. That’s why it is often, though not always, the largest of all the stones.