On September 17, 1787, the U.S. Constitutional Convention signed and adopted the Constitution at Philadelphia's Independence Hall. What exactly does that mean?
We've got the facts for you, courtesy of the Constitution page at WhiteHouse.gov.
As drafted, the Constitution's purpose was to create a government that had enough power to act on a national level, but without so much power that individuals' fundamental rights would be at risk.
The Constitution accomplished this, in part, by separating the government's power into three branches, and then including checks and balances on each of those separate powers to make sure no single branch would gain supremacy. Each branch's powers are spelled out in the Constitution, and the powers not assigned to them are reserved for the states.
This was all no coincidence -- it was based largely on the experience that the Constitutional delegates had previously had with the King of England and his powerful Parliament.
Once they'd decided on the details and language in the Constitution, the Convention got down to actually putting it on paper. (You can read more about how the Convention came about here.) The Constitution was written out by Gouverneur Morris, a delegate from Pennsylvania. This task, as it turned out, gave him control over the actual punctuation of certain clauses in the Constitution. The famous "We the People" preamble is also credited to Morris.
Finally, on September 17, 1787, 39 of the 55 delegates signed the new document -- with many of those who refused to sign objecting to its lack of a bill of rights (which would come along a few years later in the form of the first 10 amendments of the Constitution), and at least one delegate refused to sign because the document, as written, codified and protected slavery and the slave trade.
Much of the debate between the delegates was conducted in secret to make sure that those involved spoke their minds. Two plans vied for the form the new legislature would take: the Virginia Plan, supported by most of the larger states, which broke out representation based on each state's population; and the New Jersey Plan, generally preferred by smaller states, which gave each state an equal vote in Congress, regardless of population.
Ultimately, the delegates settled on "the Great Compromise," in which it was decided that the House of Representatives would represent the people according to a given state's population, and the Senate would be apportioned equally to represent the states. The plan also called for an independent judiciary, and decided that the President would be elected by the Electoral College.
They did indeed -- here's the process the founders established for amending the Constitution (a process made intentionally difficult and involved to prevent arbitrary, one-off changes):
An amendment can be proposed by a two-thirds vote of both Houses of Congress -- or, if two-thirds of the states request an amendment at a convention called for that explicit purpose. Then, the amendment has to be ratified by three-fourths of the state legislatures, or three-fourths of the conventions called in each state for ratification. The Constitution says specifically that no amendment can deny a state equal representation in the Senate without that state's consent. Since its ratification, the Constitution has been amended 27 times -- to preserve freedoms of religion, press, and speech; to protect the right to a fair and speedy trial by jury; to abolish slavery; to grant women the right to vote; and more.
Want to learn more? Head to the Government section of WhiteHouse.gov, where you'll find everything you want to know about the Constitution; the Executive, Legislative, and Judicial Branches; elections and voting; and more.
Want to read the whole thing? Take a look at this great tool from the National Archives that lets you download a high-resolution image, read a transcript, and learn more about visiting the Constitution in person.