Apportioned among the states: 

One of the points of contention at the Constitutional Convention was about how the people would be represented in Congress.  Under the Articles of Confederation, in line with the Article’s state’s rights logic, each state had equal representation.  This mode of representation had drawbacks, namely that it was less democratic as individuals would have more or less representation depending upon which state they lived in.  Citizens of a sparsely populated state (e.g., Rhode Island) had as much representation as citizens of highly populous states (e.g., Virginia).  So lightly populated states had a disproportionate say in the government.  The Virginia Plan, which set the basis for debate at the Constitutional Convention, rectified this by basing representation on population.  While the small states objected to this, the convention eventually compromised on a House of Representatives chosen based upon population and a Senate that has equal representation for each state.