The approximately 10 sq. mile historical center of Rome, the centro storico, is divided into 21 sections, or Rioni. This index has the churches listed by Rioni. This gives the churches grouped together in the same area, usually within a few blocks of each other.
The churches outside of the centro storico are grouped by the quadrant referenced to the center of Rome. These churches could be many miles apart, and will require a car or taxi to reach them.