Transepts were part of medieval Christian churches. Most churches were shaped like a cross, to remind people about Jesus’ crucifixion, and the transept is the cross-piece of the cross.
Sometimes there were big rose windows (round windows) and doors in each end of the transept.
Here is a floor plan of Chartres Cathedral, looking down at the church as if you were floating over it. You can see the nave at the west end, the apse at the east end, and the transept between them.
The place where the transept crosses the nave is the crossing, and the tower is often right above the crossing.
Transepts in cathedrals
More about Naves
More about Aisles
Bibliography and further reading about transepts and other parts of a church:
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. Easy reading.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books, and with good explanations of most architectural terms.
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