Gerrymandering exists in part because both state legislative and congressional districts are redrawn every so often so they have a uniform population size, and everyone's vote counts equally. Article 1, Section 2 of the U.S. Constitution, for example, calls for congressional representation to be apportioned according to population every 10 years, according to the results of the U.S. Census.
At least, that's how it's supposed to work.
The problem is that in most states, the process is run by the partisan politicians themselves. And they often can't resist the temptation to draw districts in ways that's advantageous to their parties. The idea is to make one party waste votes — i.e., cast them in a way that doesn't help them to win a majority of representatives, or even the amount that they should be allotted according to their percentage of the electorate [source: Cameron].
Imagine, for purposes of illustration, that a state has only 500 voters — 200 from the Yellow Party and 300 from the Orange Party. If the state is divided into five districts of 100 voters each, you'd think that would mean that the Orange Party would get be able to win three of the five seats. But if the Yellow Party has been in power for a while, it can draw districts that minimize the Orange Party's larger voting base.
One way would be to divide the state into oddly shaped districts, two of which are almost completely Orange voters. The other three would have 60 percent Yellow voters and 40 percent Orange ones. The result is that many of the Orange votes end up being wasted. The Yellow Party would waste a few votes, too, but it still would end up electing more legislators [sources: Cameron, Petry].
University of Chicago law professor Nicholas Stephanopoulos, and Eric McGhee, a research fellow at the Public Policy Institute of California, call the number of wasted votes the efficiency gap [source: Petry].
There are two main gerrymandering techniques. One is cracking, the practice of scattering an opposition party's political supporters across multiple districts, so that they don't form a majority in any of them. The other is packing, in which those partisans are jammed into a few districts (this was our Yellow and Orange Party illustration). Even though the opposition party wins overwhelmingly there, it still loses the majority of districts [source: Treleven].