in 494 BC, after the first secession of the plebs, in order to protect the interests of the plebeians against the actions of the senate and the annual magistrates, who were uniformly patrician. The ancient sources indicate the tribunes may have originally been two or five in number. If the former, the college of tribunes was expanded to five in 470 BC. Either way, the college was increased to ten in 457 BC, and remained at this number throughout Roman history. They were assisted by two aediles plebis, or plebeian aediles. Only plebeians were eligible for these offices, although there were at least two exceptions. The tribunes of the plebs had the power to convene the concilium plebis, or plebeian assembly, and propose legislation before it. Only one of the tribunes could preside over this assembly, which had the power to pass laws affecting only the plebeians, known as plebiscita, or plebiscites. After 287 BC, the decrees of the concilium plebis had the effect of law over all Roman citizens. By the 3rd century BC, the tribunes could also convene and propose legislation before the senate. Although sometimes referred to as "plebeian magistrates," technically the tribunes of the plebs were not magistrates, having been elected by the plebeians alone, and not the whole Roman people. However, they were sacrosanct, and the whole body of the plebeians were pledged to protect the tribunes against any assault or interference with their persons during their terms of office. Anyone who violated the sacrosanctity of the tribunes might be killed without penalty. This was also the source of the tribunes' power, known as ius intercessionis, or intercessio, by which any tribune could intercede on behalf of a Roman citizen to prohibit the act of a magistrate or other official. Citizens could appeal the decisions of the magistrates to the tribunes, who would then be obliged to determine the legality of the action before a magistrate could proceed. This power also allowed the tribunes to forbid, or veto any act of the senate or another assembly. Only a dictator was exempt from these powers. The tribunicia potestas, or tribunician power, was limited by the fact that it was derived from the oath of the people to defend the tribunes. This limited most of the tribunes' actions to the boundaries of the city itself, as well as a radius of one mile around. They had no