With the abolition of the Roman monarchy in 509 BC, the imperium, or executive power, of the king was divided between two annually-elected magistrates, known as praetors. In time they would come to be known as consuls, although probably not until the creation of a third, junior praetor in 367 BC. Neither consul was superior to the other, and the decisions of one could be appealed to the other (provocatio). Their insignia were the toga praetexta and the sella curulis, and each was attended by an escort of twelve lictors, each of whom bore the fasces, a bundle of rods topped by an axe; but by custom the lictors had to remove the axes from their fasces within the pomerium, the sacred boundary of Rome, to signify that the people, and not the consuls, were sovereign.
After several years, the fear of impending war with both the Sabines and the Latin League, combined with widespread suspicion that one or both of the consuls favoured the restoration of the monarchy, led to the call for a praetor maximus, or dictator ("one who gives orders"), akin to the supreme magistrate of other Latin towns. According to most authorities, the first dictator was Titus Lartius, who appointed Spurius Cassius his magister equitum.
Although there are indications that the term praetor maximus may have been used in the earliest period, the official title of the dictator throughout the history of the Republic was magister populi, or "master of the infantry". His lieutenant, the magister equitum, was the "master of the horse" (that is, of the cavalry). However, the use of dictator to refer to the magister populi seems to have been widespread from a very early period.
The appointment of a dictator involved three steps: first, the Senate would issue a decree known as a senatus consultum, authorizing one of the consuls to nominate a dictator. Technically, a senatus consultum was advisory, and did not have the force of law, but in practice it was nearly always followed. Either consul could nominate a dictator. If both consuls were available, the dictator was chosen by agreement; if they could not agree, the consuls would draw lots for the responsibility. Finally, the Comitia Curiata would be called upon to confer imperium on the dictator through the passage of a law known as a lex curiata de imperio.
A dictator could be nominated for different reasons, or causa. The three most common were rei gerundae causa, "for the matter to be done", used in the case of dictators appointed to hold a military command against a specific enemy; comitiorum habendorum causa, for holding the comitia, or elections, when the consuls were unable to do so; and clavi figendi causa, an important religious rite involving the driving of a nail into the wall of the Temple of Jupiter Optimus Maximus, as a protection against pestilence. Other reasons included seditionis sedandae causa ("to quell sedition"); ferarium constituendarum causa (to establish a religious holiday in response to a dreadful portent); ludorum faciendorum causa (to hold the Ludi Romani, or "Roman Games", an ancient religious festival); quaestionibus exercendis, (to investigate certain actions); and in one extraordinary case, senatus legendi causa, to fill up the ranks of the Senate after the Battle of Cannae. These reasons could be combined (seditionis sedandae et rei gerundae causa), but are not always recorded or clearly stated in ancient authorities, and must instead be inferred.