Importance of a keystone in an arch?

[clue.less]

this site: http://www.cpo.com/weblabs/chap3/archf.htm"

helped me to understand the forces acting on each side block an arch, but it doesn't give any information about the keystone.

Can someone explain a keystone's importance? What is its function, and how does it keep itself from falling?

 

[0xDEADBEEF]

There is nothing special about the keystone. You could also build an arch with an even number of stones. The keystone is wedged so to move downwards it would need to move the other stones out of the way (up for example), which is prevented by friction.

 

[Canticle]

It's also partly about construction. The keystone is normally the last stone to go into release the load on the formers. There are also many different shaped arches, and in many, i.e. gothic arches, the keystone takes the most compressive load, as all the load line doesn't necessarily follow down the arch soffitt.