How are voting districts drawn?

In most states, the state legislature is responsible for drawing up voting districts. This means, in effect, that the party in control of a state legislature redraws district lines in such a way as to gain political benefits. Usually this is done after a census, since districts are required by federal law and by Supreme Court decisions to be as close to equal in population as possible. This is especially true of federal Congressional districts. The principle of "one man, one vote" established in the Supreme Court case Reynolds v. Sims established this principle. So state legislatures can draw district lines in ways that benefit the majority party, a process known as "gerrymandering," but they face the constraint that districts must be equal in population. Moreover, legislatures are prohibited by federal law (the Voting Rights Act of 1965, which was severely limited in scope by the Supreme Court in a 2013 decision) from using race as a deciding factor in drawing district lines.