After the mythical expulsion of the last Etruscan King Lucius Tarquinius Superbus and the ending of the Roman Kingdom, all the powers and authority of the King were allegedly given to the newly instituted Consulship. However, it is likely that first the chief magistrates were the Praetors. The office of Consul is believed 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. Consuls had extensive competences in peacetime, administrative, legislative and judicial, and in (frequent) war time often held the highest military command(s); additional religious duties included certain rites which, as a sign of their formal importance, could only be carried out by top level state officials (compare Rex sacrorum); the reading of the auguries was an essential step before leading armies into the field.
Under the laws of the Republic, the minimum age of election to consul for patricians was 40 years of age, for plebeians 42. Two consuls were elected each year, serving together with veto power over each other's actions, a normal principle for magistratures.
In Latin, consules means "those who walk together". If a consul died during his term (not uncommon when consuls were in the forefront of battle), another would be elected, and be known as a suffect consul (cos. suff.).
According to tradition, the consulship was initially reserved for patricians and only in 367 BC the plebeians won 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 thereby elected the following year. 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; probably only the chronology has been distorted.
During times of war, the primary criterion 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.
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.
When Augustus established the Principate, he changed the political nature of the office, stripping it of most of its powers. While still a great honor — in fact invariably the constitutional head of state, hence eponymous — and a requirement for other offices, many consuls would resign part way through the year to allow other men to finish their term as suffects. Those who held the office on January 1, known as the consules ordinarii, had the honor of associating their names with that year. As a result, about half of the men who held the rank of praetor could also reach the consulship. Sometimes these suffect consuls would in turn resign, and another suffect would be appointed. This reached its extreme under Commodus, when in 190 twenty-five men held the consulship.
Emperors frequently appointed themselves, protégés, or relatives consul, even without regard to the age requirements. For example, Emperor Honorius was given the consulship at birth.
Holding the consulship was a great honor and the office was the major symbol of the still 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,
One of the reforms of Constantine I 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, the emperor of each half acquired the right of appointing one of the consuls— although one emperor did allow his colleague to appoint both consuls for various reasons. As a result, after the formal end of the Roman Empire in the West, many years would be named for only a single consul. This rank was finally allowed to lapse in the reign of Justinian I: first with the consul of Rome in 534, Decius Paulinus, then the consul of Constantinople in 541, Flavius Basilius Junior.
The highest magistrates were eponymous, i.e. each year was officially identified (like a regnal year in a monarchy) by the two Consuls' names, though there was a more practical numerical dating ab urbe condita (i.e. by the era starting with the mythical foundation year of Rome). For instance, the year 59 BC in the modern calendar was called by the Romans "the consulship of Caesar and Bibulus," since the two colleagues in the consulship were (Gaius) Julius Caesar and Marcus Calpurnius Bibulus--although Caesar dominated the consulship so thoroughly that year that it was jokingly referred to as "the consulship of Gaius and Julius".
In Latin, the ablative absolute construction is frequently used to express the date, such as "M. Messalla et M. Pupio Pisone consulibus," translated literally as "Marcus Messalla and Marcus Pupio Piso being Consuls," which appears in Caesar's De Bello Gallico.
The consular elections were typically held during July, but occasionally postponed or held earlier under special circumstances. The consuls-designate would prepare to take office during the remainder of the year and finally assume their position in the beginning of January. Thus, their ascension to office marked the beginning of each eponymous year.