The Articles of Confederation were the first Constitution of the US. The Articles created a single house of representatives, called Congress of the Confederation. It was a unicameral body.
Each state had one vote, despite having more or less population. The Congress would meet annually and when the Congress was not in session a Committee of the US would be in charge.
When the U.S Constitution was written there were many problems with representation since the Articles of Confederation. Because of this, they created a bicameral Congress. The Senate would have equal representation of states with two representatives each. The House of Representatives has a different system, the number of representatives varies from the state's population. This solved the problem with representatives in the US.
