The ceremonial boundary of an ancient Roman city.
The Romans thought the ritual for creating a pomerium was derived from the Etruscans. A bull and a cow were yoked together to a plough. The team then ploughed a furrow around the city, marking the boundary of the gods' protection.
Symbolically, the pomerium tied the city to its farming community. On a practical basis, it created the city limits, the dividing line between the urban and the rural. Certain activities were not allowed within its bounds; most prominently, the temples of "foreign gods" were not permitted inside.
If a city grew, the pomerium had to be enlarged to extend divine protection to the new areas. The pomerium of ancient Rome, which was reputed to have been created by Romulus, was enlarged at least twice (once under Claudius, once under Hadrian).