What is a groin vault?
Six-part groin vault (Abbaye aux Dames, Caen, 1050 AD)
In Romanesque churches, in the 1000s AD, architects built the stone roof with a barrel vault, or often they just had a wood roof. But wood roofs were always catching on fire and burning down the whole church, and if you put on a barrel vault the roof was so heavy that the walls had to be very thick, with only a few tiny windows, and the church looked dark.
Around 1050 AD, people began using a different kind of vault - the groin vault. It's called a groin vault because the parts meet together in a V, like the V where your legs come together. You make a groin vault by building two barrel vaults that cross each other, so that they make an X. A groin vault can be rounded, as in Romanesque churches, or pointed, as in Gothic churches.
The main advantage of the groin vault is that it takes all the weight of the roof and concentrates it on just four points at the corners of each bay (each X). If you have a buttress at each corner, you don't need to have a solid wall between those buttresses, because that wall isn't holding much weight - it can be nearly all glass windows.
Four-part groin vault (Rouen, 1200s AD)
Early medieval groin vaults, like those in the Abbaye aux Dames at Caen above, had six places where the weight came down - the corners of the X and the ends of another arch too. Laon cathedral, and Notre Dame in Paris, also used this six-part vault. But by the 1200s most churches, like Chartres or Rouen, used a four-part vault, which needed fewer buttresses and allowed for more light to enter the cathedral through bigger windows.
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. For younger kids.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books for kids, and with good explanations of most architectural terms.
Cathedral: The Story of Its Construction, by David Macaulay (1981). Beautiful drawings and clear text explain exactly how medieval craftsmen built a groined vault. Easy reading.