During the Constitutional Convention in Philadelphia, on 29 May 1787, Edmund Randolph, one of the delegates from Virginia, presented a plan created by James Madison (another delegate from Virginia) which became known as the Virginia Plan. The Articles of Confederation had produced a weak and inadequate national government and the 15 resolutions of the Virginia Plan were presented to address this problem.
One of the resolutions proposed that the government consist of three branches - legislative, executive and judicial. Another issue was that of representation. Under the Articles of Confederation, each state, regardless of size or population, had one vote in Congress. The Virginia Plan proposed that the legislative branch be compiled of representatives from each state based on the number of free inhabitants. Another plan presented was from New Jersey, which promoted a single-chamber legislature which was very similar to the Articles of Confederation. In the end, a compromise was reached which created a House of Representatives (based on population size) and a Senate (with each state being represented equally). This was called the Connecticut Compromise.
You can read the minutes of 29 May 1787 on Footnote by clicking on the image to the right.