Under the Republic
After the legendary expulsion of the last Etruscan King, Tarquin the Proud, a harsh ruler at the end of the Roman Kingdom, most of the powers and authority of the king were ostensibly given to the newly instituted consulship. This change in leadership came about when the king's son, Sextus Tarquinius, raped the wife and daughter of powerful Roman nobles. A group of nobles led by Lucius Junius Brutus, with the support of the Roman Army, expelled Tarquinius and his family from Rome in 509 BC. Originally, consuls were called praetors ("leader"), 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.
The consul 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. During the 440s, the office was quite often replaced with the establishment of the Consular Tribunes, who were elected whenever the military needs of the state were significant enough to warrant the election of more than the two usual consuls. These remained in place until the office was abolished in 367/366 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 step 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. It is thought that originally only patricians were eligible for the consulship. Consuls were elected by the Comitia Centuriata, which had an aristocratic bias in its voting structure which only increased over the years from its foundation. 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 Lex Licinia Sextia 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, according to Gelzer , only 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 ambitious Roman who chose to pursue political power and influence. When Lucius Cornelius Sulla regulated the cursus by law, the minimum age of election to consul became, in effect, 41 years of age.
Beginning in the late Republic, after finishing a consular year, a former consul would usually serve a lucrative term as a proconsul, the Roman Governor of one of the (senatorial) provinces. The most commonly chosen province for the proconsulship was Cisalpine Gaul.
Under the Empire
Although throughout the early years of the Principate, the consuls were still formally elected by the Comitia Centuriata, they were in fact nominated by the princeps. As the years progressed, the distinction between the Comitia Centuriata and the Comitia 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 "an 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 period of the High Empire (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, consuls, even without regard to the age requirements. For example, Emperor Honorius was given the consulship at birth. Cassius Dio states that Caligula intended to make his horse Incitatus consul, but was assassinated before he could do so.
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 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, as 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.
By the end of the 3rd century, much had changed. The loss of many pre-consular functions and the gradual encroachment of the equites into the traditional senatorial administrative and military functions, meant that senatorial careers virtually vanished prior to their appointment as consuls. This had the effect of seeing a suffect consulship granted at an earlier age, to the point that by the 4th century, it was being held by men in their early twenties, and possibly younger, without the significant political careers behind them that was normal previously. As time progressed, second consulates, usually ordinary, became far more common than had been the case during the first two centuries, while the first consulship was usually a suffect consulate. Also, the consulate during this period was no longer just the province of senators – the automatic awarding of a suffect consulship to the equestrian praetorian prefects (who were given the ornamenta consularia upon achieving their office) allowed them to style themselves cos. II when they were later granted an ordinary consulship by the emperor. All this had the effect of further devaluing the office of consul, to the point that by the final years of the 3rd century, holding an ordinary consulate was occasionally left out of the cursus inscriptions, while suffect consulships were hardly ever recorded by the first decades of the 4th century.
One of the reforms of Constantine I (r. 306–337) was to assign one of the consuls to the city of Rome, and the other to Constantinople. Therefore, when the Roman Empire was divided into two halves on the death of Theodosius I (r. 379–395), the emperor of each half acquired the right of appointing one of the consuls—although on occasion an emperor did allow his colleague to appoint both consuls for various reasons. The consulship, bereft of any real power, continued to be a great honor, but the celebrations attending it – above all the chariot races – had come to involve considerable expense, which only a few citizens could afford, to the extent that part of the expense had to be covered by the state. In the 6th century, the consulship was increasingly sparsely given, until it was allowed to lapse under Justinian I (r. 527–565): the western consulship lapsed in 534, with Decius Paulinus the last holder, and the consulship of the East in 541, with Anicius Faustus Albinus Basilius. Consular dating had already been abolished in 537, when Justinian introduced dating by the emperor's regnal year and the indiction. In the eastern court, the appointment to consulship became a part of the rite of proclamation of a new emperor from Justin II (r. 565–578) on, and is last attested in the proclamation of the future Constans II (r. 641–668) as consul in 632. In the late 9th century, Emperor Leo the Wise (r. 886–912) finally abolished consular dating with Novel 94. By that time, the Greek titles for consul and ex-consul, "hypatos" and "apo hypaton", had been transformed to relatively lowly honorary dignities.
In the west, the rank of consul was occasionally bestowed upon individuals by the Papacy. In 719, the title of Roman consul was offered by the Pope to Charles Martel, although he refused it. About 853, Alfred the Great, then a child aged four or five, was made a Roman consul by the Pope.