The transformation of St John Lateran, which started under pope Nicolas IV (1288–92) was the largest building project in high medieval Rome. What motives forced this pope to demolish the apses and to erect new apses, transepts and façades of the Lateran-church and of St Maria Maggiore, whose walls were sanctified by legends? This chapter focuses on the transept of St John Lateran. The form and function of the little transept aisle of Constantinian origin, excavated in the south part, is not very clear and nothing is left of a possible transept of the 12th century. This chapter explores the issue of the northern entrance towards Campus Lateranensis and the city in 1200.

In the Basilica of St John Lateran in the Middle Ages there have been many building projects, some of considerable scale, which are nonetheless not easy to identify. This chapter focusses on two examples, both of which give important insights into the building techniques employed: the lost front porch built by Pope Sergius II (844-847), which surviving illustrations indicate employed Carolingian construction technique typical for Rome, and the transept with the two bell towers attached to the northern side of the Basilica. In this second case the material evidence indicates both date of the building, in all probability from the pontificate of Nicholas IV (1288-1292), and its fine quality, appropriate to the exceptional importance of the project.