Everyone has heard of crypts--scary dark places down underground, probably with dead bodies. Well, this does describe medieval crypts, except they were not supposed to be scary.
Medieval churches were built on two layers, the ground-level church and the below-ground-level church. Main services were held in the ground-level church, which, by the twelfth century, was often very high and airy. But more private, more special services were held in the crypt. This is also where early bishops and saints were often buried.
The earliest Merovingian-era churches in France were built with crypts. In some of these churches, the crypt floor would quite literally be made of stone sarcophagi, laid next to each other. Others would have far fewer sarcophagi but would still have the tomb of a founding bishop or comparable relics. Saints' precious bones were considered safer in the crypt than up in the church.
This is the sarcophagus supposedly of Saint Benignus, first bishop-saint of Langres. He is buried in the crypt of St.-Bénigne, which was a monastery in Dijon in the Middle Ages, dedicated to him.
Even though there are extremely few medieval churches still in existence built before the eleventh century, because the High Middle Ages believed in rebuilding higher and lighter (on which see more here), many crypts under these churches are far older. Some churches still have their Merovingian-era crypts. Others had the entire church, crypt and all, redone in the ninth or tenth centuries, and then this crypt would continue to exist under a twelfth- or thirteenth-century church.
Whenever they were built, crypts would need sturdy pillars to support the church above. The pillars would often have carved capitals. This image is of a capital on a pillar in the eleventh-century crypt of St.-Bénigne of Dijon. Twelfth-century capitals were much more elegant, but down in the crypt one was back in an earlier time.