There are several types of cippus or boundary stone. They demarcated the routes of private roads or the distance of public ones, as well as delimiting the sacred boundary of Rome (the pomerium) and the banks of the Tiber. Cippi marked the course of an aqueduct and separated its space, which was to be left clear of development, from public or private land. Because these stones were progressively numbered, they also effectively served as milestones. Introduced by Augustus, their disposition was overseen by the curator aquarum, who maintained the aqueducts to ensure a sufficient supply of water to Rome, although concessions for the private consumption were the prerogative of the emperor (Frontinus, De Aquaeductu, LXXVIII.2).
No cippi record the name of an emperor later than Claudius (AD 41-54). The one above dates to AD 38-49 and was discovered in 1869 at the Villa Negroni in Rome. The inscription reads
"From here pass the conduits of three aqueducts [Aqua Marcia, Aqua Tepula, Aqua Julia]. The cippi were placed by Aulus Didius Gallus, Titus Rubrius Nepos and Marcus Cornelius Firmus, managers of the aqueducts and the water supply."
Didius Gallus, governor of Britannia (AD 52-57), is mentioned by Frontinus as one of those who preceded him in office (De Aquis, CII).
The cippus is in the Galleria Lapidaria of the Musei Capitolini (Rome).
