In the 1100s AD, architects in northern France wanted to build big impressive Gothic cathedrals. They also wanted their cathedrals to be full of light, so they would be inspiring, not dark and depressing.
What is a flying buttress?
The flying buttress would start from the places at the top of the wall where the groin vaults were directing the weight of the roof.
From there, the flying buttresses would carry the weight of the roof away from the building and down a column of stone to the ground. It wouldn’t matter what the walls were made of anymore, because they wouldn’t be carrying the weight of the roof.
Walls of glass
Once architects began to use flying buttresses in their churches, they began to make more and more of the wall out of glass, and cathedrals looked lighter and more heavenly.
Where did they use flying buttresses?
In Italy, there were hot summers. People liked their cathedrals to have a lot of stone and not so much glass, so they would be cooler in the heat.
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. Easy reading.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books, and with good explanations of most architectural terms.