Knight (Latin eques): title of members of the elite of the Roman republic. Under the empire, they were "second tier", after the senators.
In Classical Antiquity, most armies consisted of infantry. Horsemen were only employed to escort troops or at the end of battles, to attack and protect refugees. The real fighting was done by infantry. Still, ancient armies needed cavalry, and the horsemen usually belonged to the highest classes, because no one else could afford a horse. After all, horses needed to eat but were rather useless because no one knew how to harness them to a cart or plow, and as the stirrup had not yet been invented, it took a lot of time to learn to ride. Consequently, it was prestigious to own and ride a horse: you could show that you were rich and did not have to work.
Archaic Rome was probably no exception to this rule, although it is a bit puzzling that the horsemen received financial compensation to buy a horse (the equus publicus, "horse bought by the commonwealth"). Why should the rich be subsidized? A possible explanation is that Rome needed a lot of horsemen and had only a few rich people; alternatively, the rich had found a way to become even richer.
In our sources, the knights are both a military and a political group. It is said that king Servius Tullius divided the Roman nation into centuries, which were not only units of soldiers on the battlefield but also voting units in the comitia centuriata. The Roman historian Livy offers a description of a complex system with 18 centuries of cavalry, 170 centuries of infantry, and 2 centuries of engineers. When the centuries came together to vote, the cavalry centuries cast their 18 votes first, followed by the 172 remaining centuries, and one additional vote for those who were too poor to serve in the army but still had a political vote. (Although the fact that the people were divided belongs to the age of kings, it is likely that these specific numbers date back to the fourth century BCE.)
In the late third century, Rome started to rely upon its allies as cavalry men, and in the second century, this development continued. As a result, the equestrian centuries lost their military function. The elite of the Roman empire still called itself "knights", but like a knighthood today, it did not mean that one really fought on horseback.
At the same time, the Roman elite slowly started to change. Always, the knights had been wealthy and had governed the empire. Words like "knight" and "senator" had been synonyms. However, the Senate increasingly became a body of former magistrates, and although other rich people could still be invited to join the discussions, the families that had produced magistrates tended to intermarry, thereby creating a senatorial elite within the old, equestrian elite.
So Rome started to have a dual elite. There was a class of senators, which was just as rich as the knights; but senators tended to monopolize the government and started to act as if they were an elite within the elite. Senators had to behave according to a strict code of conduct, and commercial incomes were officially taboo (Lex Claudia). For the other rich people, this taboo was less rigid, and the knights often invested money in tax farming companies. As a result, tensions arose between the elite of magistrates and the elite of bankers. After all, the knights wanted to make as much money as possible from their tax farming companies and were simply extorting the provinces, whereas the senators governed the provinces and noticed that overtaxing caused rebellions.
In the late second century, the latent tensions within Rome's double elite were used by a tribune named Gaius Sempronius Gracchus. He wanted to reform Roman society, but the elite had always been able to overcome any opposition. Therefore, Gracchus decided to divide the elite first, and proposed to make the knights jurors in extortion trials. They could judge their own conduct in the provinces, much against the wishes of the senators. From now on, the senators and knights existed as independent classes with different rights, obligations, and interests.
Still, in the age of the Roman civil wars, the senatorial and equestrian orders often collaborated. Control of the courts and financial management of the provinces were two fields in which they sometimes clashed, but mostly, these rich people had the same interests. Knights and senators were usually no proponents of social change or revolution. Besides, a knight who obtained a political office (e.g., the quaestorship) would become a senator, whereas the son of a senator who failed to obtain office, still was a knight. The two classes belonged together.
During the reign of the emperor Augustus, the two orders were for the first time officially defined. One could become a knight when one had some 400,000 sesterces; a senator needed a million. Of course one also needed to be registered in one of the six senatorial or twelve equestrian centuries, and the censor (usually the emperor) wrote down the names of worthy people on a list (adlectio). Our sources make a difference between knights with and without an equus publicus, but the difference is not fully understood by modern scholars.
What is clear, however, is that Augustus wanted to recreate the equestrian order as military class, with special rites in the temple of Castor and Pollux. This policy, however, failed, because there was another development that was more important.
The empire needed a bureaucracy, but no freeborn Roman would serve another man. As a result, freedmen became very important during the reigns of Claudius (41-54) and Nero (54-68). This was not an acceptable solution. These freedmen could become very influential and senators did not appreciate it when a former slave had greater power than they had. From the reign of the emperor Vitellius, who ruled in the year 69, on, equestrian procurators started to serve as heads of the great ministries of the Roman government. The knights developed into a bureaucratic and practical elite. The senators still occupied the representative offices and acted as governors in the major provinces, but the knights did the real work.
Officially, the knights were the second tier of the elite. In the theater and amphitheater, they occupied ranks behind the senators. This made the knights harmless and, consequently, suitable for really important offices: a senator who served as praetorian prefect or prefect of Egypt might start to dream of making himself emperor, so these offices were reserved for knights. In the late second century, the emperor Commodus and his successor Septimius Severus increasingly relied upon the equestrian order. Legions, for example, received knights as commanders, and the newly conquered provinces of Mesopotamia were governed by equestrian prefects.
Senatorial authors like Cassius Dio did not appreciate it, but it was inevitable. Strong tribes threatened the Roman frontiers, and it would have been irresponsible to hand over the command of the armies to senators.
Knights were allowed to wear golden rings and white tunicas with a narrow purple stripe.