Ancient Rome operated as a two class society consisting of Patricians and Plebians. Patricians were the aristocratic class and Plebians were all Roman citizens who were not Patrician. The word Patrician comes from "Patres", the plural of father, a term used to designate the first members of the Senate. The term Plebian means "of the common people".
In the beginning of the Republic, the Patrician class controlled the administration of laws, all political offices, and the state treasury. Then, over time, agitation by the lower class broke down the control of the Patricians, and extended rights to the lower class. The Patricians agreed to display a list laws in the Forum to prevent indescriminate application of justice, and courts were made more equitable. High elected offices were opened up to the Plebian class and they began to have more of a say in government.
The Republic included, in its system of government, more than one People's Assembly to vote on and pass new laws. One of these assemblies was modeled on the structure of the Army, and the Knights (members of the cavalry) became the most influential of its members. Later the Knights evolved into the Equestrian order - a third social class, above the Plebians but below the Patricians.
Colonial America had an informal class system of its own. The rich who came over from Europe considered themselves patrician, while the poorest of the immigrants, the indentured servants, were little more than slaves. Fortunately, the hardships of the wilderness and the independent spirit of the colonists, eventually eroded away any pretense of a class system in America. Those who were clever could accumulate wealth for themselves and become independent of the rich, and the political system contained assembly bodies with independent minded people who were not willing to live under the power of either the wealthy class or the crown.
The American middle class is mich like the Equestrian class of Rome, because it fills the void between rich and poor: a class made up of those citizens who, by their own skill, can rise above humble roots.