Why did the Framers of the Constitution create the legislative branch with a bicameral structure

All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives.

— The Constitution, Article I Section I - The Legislature

Congress is divided into two institutions: the House of Representatives and the Senate. The two houses of Congress have equal but unique roles in the federal government. While they share legislative responsibilities, each house also has special constitutional duties and powers.

To balance the interests of both the small and large states, the Framers of the Constitution divided the power of Congress between the two houses. Every state has an equal voice in the Senate, while representation in the House of Representatives is based on the size of each state’s population.

Known as the Great (or Connecticut) Compromise, this plan for representation in Congress was introduced by Connecticut delegates to the 1787 Constitutional Convention, Roger Sherman and Oliver Ellsworth.

The term “bicameral legislature” refers to any lawmaking body of government that consists of two separate houses or chambers, such as the House of Representatives and the Senate that make up the United States Congress.

• Bicameral systems separate the legislative branch of government into two separate and distinct divisions or “chambers,” as opposed to unicameral systems which employ no such division.
• The U.S. bicameral system—the Congress—is composed of the House of Representatives and the Senate.
• The number of members of the House of Representatives is based on each state’s population, while the Senate is composed of two members from each state.
• Each chamber of a bicameral legislature has different powers in order to ensure fairness through checks and balances within the system.

Indeed, the word “bicameral” comes from the Latin word “camera,” which translates to “chamber” in English.

Bicameral legislatures are intended to provide representation at the central or federal level of government for both the individual citizens of the country, as well as the legislative bodies of country’s states or other political subdivisions. About half of the world’s governments have bicameral legislatures.

In the United States, the bicameral concept of shared representation is exemplified by the House of Representatives, whose 435 members look after the interests of all residents of the states they represent, and the Senate, whose 100 members (two from each state) represent the interests of their state governments. A similar example of a bicameral legislature can be found in the English Parliament’s House of Commons and House of Lords.

There have always been two differing opinions on the effectiveness and purpose of bicameral legislatures:

Bicameral legislatures enforce an effective system of checks and balances preventing the enactment of laws unfairly impacting or favoring certain factions of the government or the people.

The procedures of bicameral legislatures in which both chambers must approve legislation often result in complications slowing or blocking the passage of important laws.

Currently, about 41% of governments worldwide have bicameral legislatures and about 59% employ various forms of unicameral legislatures. Some countries with bicameral legislatures include Australia, Brazil, Canada, the Czech Republic, Germany, India, the United Kingdom, Ireland, the Netherlands, Russia, and Spain. In countries with bicameral legislatures, the size, length of term in office, and manner of election or appointment for each chamber will vary. Growing somewhat in popularity during the 20th century, unicameral legislatures have more recently been adopted in countries like Greece, New Zealand, and Peru.

The bicameral legislature in the United Kingdom—the Parliament—originally formed in 1707, consists of the House of Lords and the House of Commons. The upper House of Lords represents a smaller, more elite social class, while the lower House of Commons represents a larger, less-exclusive class. While the U.S. Senate and House were modeled after the British House of Lords and House of Commons, America’s bicameral legislature was designed to represent residents in different geographic locations rather than different socio-economic classes.

In the bicameral U.S. Congress, those complications and blocking of the legislative process can happen at any time but are far more likely during periods when the House and Senate are controlled by different political parties.

So why do we have a bicameral Congress? Since members of both chambers are elected by and represent the American people, wouldn't the lawmaking process be more efficient if bills were considered by only one “unicameral” body?

While it is at times truly clumsy and overly time-consuming, the bicameral U.S. Congress works today exactly the way a majority of the framers of the Constitution envisioned in 1787. Clearly expressed in the Constitution is their belief that power should be shared among all units of government. Dividing Congress into two chambers, with the positive vote of both required to approve legislation, is a natural extension of the framers' concept of separation of powers to prevent tyranny.

The provision of a bicameral Congress didn't come without debate. Indeed, the question almost derailed the entire Constitutional Convention. Delegates from the small states demanded that all states be equally represented in Congress. The large states argued that since they had more voters, representation should be based on population. After months of great debate, delegates arrived at the “Great Compromise,” under which the small states got equal representation (two Senators from each state) in the Senate, and the large states got proportional representation based on population in the House.

But is the Great Compromise really all that fair? Consider that the largest state–California—with a population about 73 times larger than that of the smallest state—Wyoming—both get two seats in the Senate. Thus, it can be argued that an individual voter in Wyoming wields about 73 times more power in the Senate than an individual voter in California. Is that “​one man—one vote?”

Have you ever noticed that major bills are often debated and voted on by the House in a single day, while the Senate's deliberations on the same bill take weeks? Again, this reflects the Founding Fathers' intent that the House and Senate were not carbon-copies of each other. By designing differences into the House and Senate, the Founders assured that all legislation would be carefully considered, taking both the short and long-term effects into account.

The Founders intended that the House be seen as more closely representing the will of the people than the Senate.

To this end, they provided that members of the House—U.S. Representatives—be elected by and represent limited groups of citizens living in small geographically defined districts within each state. Senators, on the other hand, are elected by and represent all voters of their state. When the House considers a bill, individual members tend to base their votes primarily on how the bill might impact the people of their local district, while Senators tend to consider how the bill would impact the nation as a whole. This is just as the Founders intended.

In contrast to bicameral legislatures, with two chambers, unicameral legislatures consist of only one house or assembly that legislates and votes as one. 

Bicameral legislatures, like the United States Congress, are typically justified as an extension of the larger principle of checks and balances. In theory, bicameral legislatures are desirable because they avoid hasty and extreme legislation by ensuring more thoughtful deliberation. Because of their requirement for mutual agreement of both chambers, bicameral legislatures are considered to be more likely than unicameral legislatures to result in a measured compromise between different political and social ideals.

Despite occasional opposition to them, bicameral national government legislatures remained prevalent throughout the 20th century. Following patterns established in the 19th century, unicameral councils or commissions came to predominate in American cities and counties. During the early 1900s, dissatisfaction with the sluggishness of bicameral American state legislatures led to numerous proposals for single-chamber systems. Today, however, Nebraska remains the only U.S. state with a unicameral legislature.

Following World War II, constitutional trends reflected a growing preference for unicameral systems among non-federal countries in which the national governments maintain exclusive power over all geographic areas. Many European countries and several Latin American countries established unicameral legislatures. Some modern countries with unicameral legislatures include Cuba, Honduras, Guatemala, Bulgaria, Denmark, Hungary, Monaco, Ukraine, Serbia, Turkey, and Sweden.

In Britain, where the Parliament’s House of Commons has become far more powerful than the House of Lords, and in France where the Senate, under the 1958 foundation of the Fifth Republic, remains virtually impotent, the governments effectively operate, on the unicameral principle. A unitary system of government does not imply a unicameral legislature. Modern constitutional countries often retain two-chamber legislatures even though true bicameralism has declined overall.

All members of the House are up for election every two years. In effect, they are always running for election. This ensures that members will maintain close personal contact with their local constituents, thus remaining constantly aware of their opinions and needs, and better able to act as their advocates in Washington. Elected for six-year terms, Senators remain somewhat more insulated from the people, thus less likely to be tempted to vote according to the short-term passions of public opinion.

By setting the constitutionally-required minimum age for Senators at 30, as opposed to 25 for members of the House, the Founders hoped Senators would be more likely to consider the long-term effects of legislation and practice a more mature, thoughtful, and deeply deliberative approach in their arguments. Setting aside the validity of this "maturity" factor, the Senate undeniably does take longer to consider bills, often brings up points not considered by the House, and just as often votes down bills passed easily by the House.

A famous (though perhaps fictional) quip often quoted to point out the differences between the House and Senate involves an argument between George Washington, who favored having two chambers of Congress, and Thomas Jefferson, who believed a second legislative chamber unnecessary. The story goes that the two Founding Fathers were arguing the issue while drinking coffee. Suddenly, Washington asked Jefferson, "Why did you pour that coffee into your saucer?" "To cool it," replied Jefferson. "Even so," said Washington, "we pour legislation into the senatorial saucer to cool it."