Double Aisles
Some big basilicas and churches had two aisles instead of one. You can see a picture of the big cathedral at Pisa (the Duomo), which has two aisles. See the two rows of columns?
Double aisles were needed for very big churches. The extra row of columns helped to hold up the heavy stone roof.
Floor plan of Pisa Cathedral
Here is a floor plan of Pisa Cathedral, looking down at the church as if you were floating over it. You can see the four rows of columns, and the side aisles, and the apse at the other end.
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).
