Plan of "Old" St. Peter's Basilica.
One of the first Christian churches in Rome, Old St. Peter's followed the plan of the Roman basilica and added a transept (labeled "Bema" in this diagram) to give the church a cruciform shape.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"640px-Old_St_Peters_Basilica_plan.png."
https://commons.wikimedia.org/w/index.php?curid=10899121
Wikimedia Commons
Public domain.