As we set upon preparation for Lent, some of our readers may find it of interest that this practice originated, it is said, out of a desire to provide for a more pious and spiritual alternative to Rome's 'carnival.' The traditional route in question involved a walkabout the city, touching on the basilicas of St. John Lateran, St. Peter's Basilica, St. Paul Outside the Walls, St. Mary Major, St. Lawrence Outside the Walls, St. Sebastian Outside the Walls and Santa Croce in Gerusalemme.
It is worth noting that the four major papal basilicas found in this list were already designated as "pilgrim churches" in their own right be various popes in the fourteenth century, but our focus here is on the tradition of a pilgrimage made to these seven basilicas successively.
The idea behind the pilgrimage was a devotion to the early Roman Christian saints and martyrs, of which these seven basilicas are, of course, closely affiliated. In this regard, the itinerary in question is founded in 'Romanitas.'
Evidently the four major papal basilicas are well enough known to all, but if you're wondering about the other three are on the pilgrimage itinerary, here they are.
San Lorenzo:
Santa Croce: