Equites
The Equites (Latin: eques nom. singular; sometimes called "knights" in modern times because of the involvement of horses) constituted the lower of the two aristocratic classes of ancient Rome, ranking below the patricians (patricii), a hereditary caste that monopolized political power during the regal era (753 to 509 BC) and during the early Republic (to 338 BC). A member of the equestrian order was known as an eques (plural: equites).
During the Roman kingdom and the 1st century of the Republic, legionary cavalry was recruited exclusively from the ranks of the patricians, who were expected to provide 6 centuriae of cavalry (300 horses for each consular legion). Around 400 BC, 12 more centuriae of cavalry were established and these included non-patricians (plebeians). Around 300 BC the Samnite Wars obliged Rome to double the normal annual military levy from 2 to 4 legions, doubling the cavalry levy from 600 to 1,200 horses. Legionary cavalry started to recruit wealthier citizens from outside the 18 centuriae. These new recruits came from the First Class of commoners in the centuriate organisation and were not granted the same privileges.
By the time of the Second Punic War (218–202 BC), all the members of the First Class of commoners were required to serve as cavalrymen. The presence of equites in the Roman cavalry diminished steadily in the period 200–88 BC as only equites could serve as the army's senior officers; as the number of legions proliferated fewer were available for ordinary cavalry service. After c. 88 BC, equites were no longer drafted into the legionary cavalry, although they remained technically liable to such service throughout the Principate era (to AD 284). They continued to supply the senior officers of the army throughout the Principate.
With the exception of the purely hereditary patricians, the equites were originally defined by a property threshold. The rank was passed from father to son, although members of the order who at the regular quinquennial census no longer met the property requirement were usually removed from the order's rolls by the Roman censors. In the late Republic, the property threshold stood at 50,000 denarii and was doubled to 100,000 by the emperor Augustus (sole rule 30 BC – AD 14) – roughly the equivalent to the annual salaries of 450 contemporary legionaries.