What is the Difference between X-Y Coincident Pairs and Blumlein Pairs?

X-Y coincident pair and Blumlein pair microphone placement both use two microphones placed close together with their capsules at a 90-degree angle from each other. The difference is that Blumlein pair microphone placement uses two bi-directional (figure-of-8) microphones, so that one mic picks up the front left and back right of the room, and the other mic picks up the front right and back left of the room, where X-Y coincident technique uses two cardioid microphones. Blumlein pair miking is particularly interesting if you are recording in a really nice sounding room, and want to capture the space.