The pomerium or pomoerium (Latin, from post + moerium > murum, "wall"), was the sacred boundary of the city of Rome. In legal terms, Rome existed only within the pomerium; everything beyond it was simply territory (ager) belonging to Rome.
Location and extensions 
Tradition maintained that the pomerium was the original line ploughed by Romulus around the walls of the original city, and that it was inaugurated by Servius Tullius. The legendary date of its demarcation, 21 April, continued to be celebrated as the anniversary of the city’s foundation.
The Roman historian Livy writes, in his Ab urbe condita, that although the etymology of the word pomerium (post – moerium) implies a meaning of "beyond the wall", the pomerium was in fact an area of ground both on the inner and outer sides of the city wall. According to Livy, it was an Etruscan tradition to consecrate this area by augury, which in part had the purpose of preventing buildings from being erected close to the wall (although he writes that in his time, houses were in fact built against the wall). Livy also writes that it was unlawful to inhabit or to farm the area of the pomerium.
The pomerium did not follow the line of the Servian walls, and remained unchanged until the Dictator Lucius Cornelius Sulla, in a demonstration of his absolute power, expanded it in 80 BC. Several white marker stones (known as cippi) commissioned by Claudius have been found in situ and several have been found away from their original location. These stones mark the boundaries and relative dimensions of the pomerium extension by Claudius. This extension is recorded in Tacitus. Aulus Gellius also reports extensions by Caesar Augustus, Nero, and Trajan, but no other written or archaeological evidence supports this.
The pomerium was not a walled area (unlike the Chinese Forbidden City), but rather a legally and religiously defined one marked by cippi. It encompassed neither the entire metropolitan area nor even all the proverbial Seven Hills (the Palatine Hill was within the pomerium, but the Capitoline and Aventine Hills were not). The Curia Hostilia and the well of the Comitium in the Forum Romanum, two extremely important locations in the government of the city-state and its empire, were located within the pomerium. The Temple of Bellona was beyond the pomerium.
Associated restrictions 
- The magistrates who held imperium did not have full power inside the pomerium. They could have a citizen beaten, but not sentenced to death. This was symbolised by removing the axes from the fasces carried by the magistrate's lictors. Only the dictator's lictors could carry fasces containing axes.
- It was forbidden to bury the dead inside the pomerium. During his life, Julius Caesar received in advance the right to a tomb inside the pomerium, but his ashes were actually placed in his family tomb. However, Trajan's ashes were interred after his death in AD 117 at the foot of his Column, which was within the pomerium.
- Provincial promagistrates and generals were forbidden from entering the pomerium, and resigned their imperium immediately upon crossing it (as it was the superlative form of the ban on armies entering Italy). Ceremonies of triumph, in which an army would march through the city in celebration of a victory, were an exception to this rule, although a general could only enter the city on the very day of his triumph, and would be required to wait outside the pomerium with his troops until that moment. Under the Republic, soldiers also lost their status when entering, becoming citizens: thus soldiers at their general's triumph wore civilian dress. The Comitia Centuriata, one of the Roman assemblies, consisting of centuriae (voting units, but originally military formations within the legions), was required to meet on the Campus Martius outside the pomerium.
- Pompey's Theatre, where Julius Caesar was murdered, was outside the pomerium and included a chamber where the Senate could meet allowing the attendance of any senators who were forbidden to cross the pomerium and thus would not have been able to meet in the Curia Hostilia.
- Weapons were banned inside the pomerium for religious and traditional reasons. Praetorian guards were allowed in only in civilian dress (toga), and were then called collectively cohors togata. But it was possible to sneak in daggers (the proverbial weapon for political violence, see sicarius). Since Julius Caesar's assassination occurred outside this boundary, the senatorial conspirators could not be charged with sacrilege for carrying weapons inside the sacred city.
See also 
- Pomerium, Encyclopedia Britannica. Retrieved 24 January 2012.
- Livy, Ab urbe condita, I.44
- Beard, Mary; North, John; Price, Simon (1998). Religions of Rome Volume 1: A History. Cambridge: Cambridge University Press. pp. 177–180. ISBN 978-0-521-31682-8.
- Epitome de Caesaribus 13.11; Eutropius 8.5.
- Samuel Ball Platner, A Topographical Dictionary of Ancient Rome: Pomerium
- Smith's Dictionary of Greek and Roman Antiquities: Pomoerium
- Transactions of the American Philological Association Alternate etymology: pro-murium