Consul was the highest elected office of the Roman Republic, which became an
appointive office under the Empire.
Under 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, they
served together with veto power over each other's actions, and the year of
their service was known by their names. For instance, the year we commonly
call 59 BC was called by the Romans "the year of Caesar and Bibulus," since
the two colleagues in the consulship were Julius Caesar and Marcus
"Consules" in Latin means "those who walk together".
When Augustus established the Empire, he changed the nature of the office,
stripping it of most if not all of its powers. While still a great honor,
and a requirement for other offices, many consuls during his long rule would
resign part way through the year, to allow other men to hold the fasces, or
become consules suffecti. 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.
Another change under the Empire was that Emperors frequently appointed
themselves, proteges, or relatives without regard to the age requirements.
For example, Honorius was conferred the consulship upon his birth.
Holding the consulate was apparently such an honor that 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
One of the reforms Constantine I made 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 only be named for a single consul. This rank was finally
allowed to lapse in the reign of Justinian: first with the Roman consul in
534, Decimus Theodorius Paulinus, then the consul of Constantinople in 541,
Flavius Basilius Junior.