Aisle
An aisle (pronounced EYE-yull) is the part of a Roman basilica or a medieval Christian church that goes down the side of the nave, between the columns and the wall.
Usually people stood mainly in the nave, and they used the aisles for walking around, so they wouldn't disturb the people who were participating in the Mass. Sometimes there were people selling snacks or souvenirs in the aisles, too. On a holiday if the church was very crowded, people would stand in the aisles, too.

Floor plan of Chartres Cathedral
Here is a floor plan of Chartres Cathedral, looking down at the church as if you were floating over it. You can see the two rows of columns, and the side aisles, and the apse at the other end.
Some big churches had double aisles - two aisles next to each other, with a row of columns separating them. Check out a church with double aisles.
To find out more about medieval churches and their aisles, check out these books from Amazon.com or from your library:
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.
City: A Story of Roman Planning and Construction, by David Macaulay (1983).


