In the earliest period of Roman history the people were divided into two classes. The patricians were the hereditary nobility who were entitled to hold public office. The plebeians (or plebs) were the common people (hence the English word 'plebian' for a common or low-born person, and 'plebiscite' for a vote by all the people of the nation). The constitution was amended in 368-367 BC, however, to allow plebs to be elected as well, and a pleb elected consul was automatically elevated to the nobility and named novus homo, "new man."
The term equestrian originally referred to any Roman wealthy enough to afford to serve as a cavalryman, i.e., a horse-mounted soldier. As they grew in economic strength, equestrians who choose not to serve in public office were permitted to form corporations to provide services to the state. The modus operandi was for the corporation to tender a figure for the services to the state financial censors. If the revenue from the services exceeded the estimates the corporation was permitted to keep it as their profit. If the revenue fell short, then it was for the corporation to make up the short fall.
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