A third aspect of the core value of constitutional government is bicameralism--the division of the legislative branch into two chambers.
The idea of a two house legislative body is an old one in the West. Most of the European monarchies at the time we won our independence boasted two house legislatures in addition to hereditary monarchy. They went by different names: cortes in Spain, diet in the Germanic principalities, estates in France, and--more familiarly to Americans--parliament in Britain. Of course, the Europeans thought about their constituent assemblies differently than we think about ours. In 18th century Europe, the two houses represented different orders in society. The upper houses represented the nobility; the lower houses represented everyone else. For example, in Britain the House of Lords was filled with titled nobility (Barons, Dukes, etc.) who inherited their position and who in some sense represented Britain's nobility. The House of Commons was filled with land owning gentry or knights from the larger cities who represented the common realm.
According to the political thinking behind the idea of two houses, they worked together to seek the common good of the people. If one or the other house tried to pursue a path that advantaged it own particular order, the other house could "check and balance" it. This idea went back to Polybius and Aristotle. This political thinking was only a theory, however. In an age of absolute monarchs, most European kings ruled; only a small or even nonexistent role played by the legislature. The kings of France, for example, refused to call the estates general into session for decades.
British settlers to North American duplicated this pattern as they erect their first provincial governments. Most colonies had a lower house elected by the people, roughly imitating the British House of Commons. Each also had an appointed governor's council, roughly imitating the British House of Lords. The governmental structures existed only as a pale imitation. No hereditary nobility put down roots in the colonies.
Many state constitutions, save that of Pennsylvania, adopted two house legislatures after the revolution. They experienced theoretical difficulties, however, justifying a two house legislature in a nation that lacked the traditional social orders of Europe. (Pennsylvanians argued that we have not aristocratic order and all American enjoyed the equal status as citizens sharing the same interests. So why have a two house legislature?)
The original national government under the Articles of Confederation maintained a one house legislature.
The Constitution of 1787 created a two house legislature, commonly known as bicameralism.
The exact form resulted from political compromise. The original Virginia Plan drafted by James Madison and submitted to the constitutional convention called for a two house legislature, in both of which representation from the states rested on population. This gave the states with larger populations more representation. More interestingly, Madison apparently tried to keep some semblance of the idea of an aristocracy. In his original plan, the lower house (House of representatives) elected the upper house (the Senate). This provided a kind of filtration of public opinion as voters elected for the membership of the House of Representatives, who in turn voted for the membership of the Senate. Instead of the inherited aristocracy of the upper houses of the European constituent assemblies, Madison hoped for a "natural aristocracy" of talent and virtue.
Madison's vision fell by the wayside, however, as a result of political compromise. The states with small populations insisted on equal representation as currently practiced under the Articles of Confederation. A compromise decided that in the House of Representatives, representation would be based upon population; in the Senate, representation would be based upon the equality of each state.
In the Federalist 62, Madison asserted that this meant that our general government was part national and part federal. It was national in that each state enjoyed representation based upon its population in the House of Representatives; it was federal in that each state enjoyed equal representation in the Senate. Under this scheme, no legislation passed "without the concurrence first of the majority of the people, and then of the majority of the states."
This introduces that idea of "checks and balances"--not between branches of the government as we learned in high school civics-- but between the two houses of the national legislature. In that same edition of the Federalist, Madison admitted that the system created a "check on legislation" that may in some instances be beneficial but in other instances be detrimental to the common good. Often however, the "checks and balances" served to insulate the government from momentary and transient passions that might overwhelm the House but be restrained by the Senate.
Sometimes these leads to so-called "government gridlock." Rather than seeing gridlock as some flaw in the system, one needs reminding that this is the system.
Until a clear consensus among voters forms about the direction our national policy ought to take, the Congress will experience "gridlock" or, as the founders called it, "checks and balances."