The consuls were the two highest elected public officials of the Roman Republic (c. 509 BC to 27 BC). Romans considered the consulship the second-highest level of the cursus honorum—an ascending sequence of public offices to which politicians aspired—after that of the censor, which was reserved for former consuls. Each year, the centuriate assembly elected two consuls to serve jointly for a one-year term. The consuls alternated each month holding fasces (taking turns leading) when both were in Rome. A consul's imperium (military power) extended over Rome and all its provinces.
Having two consuls created a check on the power of any one individual, in accordance with the republican belief that the powers of the former kings of Rome should be spread out into multiple offices. To that end, each consul could veto the actions of the other consul.
After the establishment of the Empire (27 BC), the consuls became mere symbolic representatives of Rome's republican heritage and held very little power and authority, with the Emperor acting as the supreme authority.
According to Roman tradition, after the expulsion of the last king, Tarquin Superbus, the powers and authority of the king were given to the newly instituted consulship. Originally, consuls were called praetors ("leaders"), referring to their duties as the chief military commanders. By at least 300 BC the title of consul became commonly used. Ancient writers usually derive the title consul from the Latin verb consulere, "to take counsel", but this is most likely a later gloss of the term, which probably derives—in view of the joint nature of the office—from con- and sal-, "get together" or from con- and sell-/sedl-, "sit down together with" or "next to". In Greek, the title was originally rendered as στρατηγὸς ὕπατος, strategos hypatos ("the supreme general"), and later simply as ὕπατος (hypatos).
The consulship was believed by the Romans to date back to the traditional establishment of the Republic in 509 BC, but the succession of consuls was not continuous in the 5th century BC, when the consulship was supposedly replaced with a board of consular tribunes, which was elected whenever the military needs of the state were significant enough to warrant the election of more than the usual two consuls. These remained in place until the office was abolished in 367 BC and the consulship was reintroduced.
Consuls had extensive powers in peacetime (administrative, legislative, and judicial), and in wartime often held the highest military command. Additional religious duties included certain rites which, as a sign of their formal importance, could only be carried out by the highest state officials. Consuls also read auguries, an essential religious ritual, before leading armies into the field.
Two consuls were elected each year, serving together, each with veto power over the other's actions, a normal principle for magistracies. They were elected by the comitia centuriata, which also elected praetors and censors. However, they formally assumed powers only after the ratification of their election in the older comitia curiata, which granted the consuls their imperium by enacting a law, the lex curiata de imperio.
If a consul died during his term (not uncommon when consuls were in the forefront of battle) or was removed from office, another would be elected by the comitia centuriata to serve the remainder of the term as consul suffectus (suffect consul). A consul elected to start the year, called a consul ordinarius (ordinary consul), held more prestige than a suffect consul, partly because the year would be named for ordinary consuls (see consular dating).
According to tradition, the consulship was initially reserved for patricians and only in 367 BC did plebeians win the right to stand for this supreme office, when the Licinio-Sextian rogations provided that at least one consul each year should be plebeian. The first plebeian consul, Lucius Sextius, was elected the following year. Nevertheless, the office remained largely in the hands of a few families, as only about fifteen novi homines ("new men" with no consular background) were elected to the consulship until the election of Cicero in 63 BC. Modern historians have questioned the traditional account of plebeian emancipation during the early Republic (see Conflict of the Orders), noting for instance that about thirty percent of the consuls prior to Sextius had plebeian, not patrician, names. It is possible that only the chronology has been distorted, but it seems that one of the first consuls, Lucius Junius Brutus, came from a plebeian family. Another possible explanation is that during the 5th-century social struggles, the office of consul was gradually monopolized by a patrician elite.
During times of war, the primary qualification for consul was military skill and reputation, but at all times the selection was politically charged. With the passage of time, the consulship became the normal endpoint of the cursus honorum, the sequence of offices pursued by the Roman who chose to pursue a political career. When Lucius Cornelius Sulla regulated the cursus by law, the minimum age of election to consul became 43 or 42 years old. This age requirement was later changed to 32 during the Empire.
Although throughout the early years of the Principate the consuls were still formally elected by the comitia centuriata, they were de facto nominated by the princeps. As the years progressed, the distinction between the comitia centuriata and the comitia populi tributa (which elected the lower magisterial positions) appears to have disappeared, and so for the purposes of the consular elections, there came to be just a single "assembly of the people" which elected all the magisterial positions of the state, while the consuls continued to be nominated by the princeps.
The imperial consulate during the Principate (until the 3rd century) was an important position, albeit as the method through which the Roman aristocracy could progress through to the higher levels of imperial administration—only former consuls could become consular legates, the proconsuls of Africa and Asia, or the urban prefect of Rome. It was a post that would be occupied by a man halfway through his career, in his early thirties for a patrician, or in his early forties for most others. Emperors frequently appointed themselves, or their protégés or relatives, as consuls, even without regard to the age requirements. Caligula once said that he would appoint his horse Incitatus consul, which was probably a joke intended to belittle the Senate's authority.
The need for a pool of men to fill the consular positions forced Augustus to remodel the suffect consulate, allowing more than the two elected for the ordinary consulate. During the reigns of the Julio-Claudians, the ordinary consuls who began the year usually relinquished their office mid-year, with the election for the suffect consuls occurring at the same time as that for the ordinary consuls. During reigns of the Flavian and Antonine emperors, the ordinary consuls tended to resign after a period of four months, and the elections were moved to 12 January of the year in which they were to hold office. Election of the consuls were transferred to the Senate during the Flavian or Antonine periods, although through to the 3rd century, the people were still called on to ratify the Senate's selections. The emperor did not assume the consulship of every year of his reign, but did nominate himself multiple times; Augustus was consul 13 times, Domitian 17, and Theodosius II 18.
The proliferation of suffect consuls through this process, and the allocation of this office to homines novi tended, over time, to devalue the office. However, the high regard placed upon the ordinary consulate remained intact; it was one of the few offices that one could share with the emperor, and during this period it was filled mostly by patricians or by individuals who had consular ancestors. If they were especially skilled or valued, they may even have achieved a second (or rarely, a third) consulate. Prior to achieving the consulate, these individuals already had a significant career behind them and would expect to continue serving the state, filling in the post upon which the state functioned. Consequently, holding the ordinary consulship was a great honor, and the office was the major symbol of the still relatively republican constitution. Probably as part of seeking formal legitimacy, the break-away Gallic Empire had its own pairs of consuls during its existence (260–274). The list of consuls for this state is incomplete, drawn from inscriptions and coins.