Each year, the citizens of Rome elected two consuls to serve jointly for a one-year term. The consuls alternated in holding imperium each month, and a consul's imperium extended over Rome, Italy, and the provinces. However, 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.
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[who?], 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.