Political parties use gerrymandering to give themselves a big edge on Election Day. Here's everything you need to know:

What is gerrymandering?
It's a tactic used by political parties to redraw voting districts to give themselves an electoral advantage. Whether Republicans or Democrats control the process in a given state, the trick is to create irregularly shaped districts that segregate as many of the opposition's supporters as possible into a small handful of seats — leaving their own candidates with a much better chance of winning everywhere else. To simplify, in a state with 100,000 Democratic and 100,000 Republican voters and six districts, a GOP legislature would group 80,000 Democrats into two districts. That would leave just 20,000 Democrats spread over the other four districts, which the GOP could then easily win. This process has left most states with oddly shaped districts, often with strips of land jutting out in several directions. It's perfectly legal, unless it can be proved that districts are deliberately drawn to disenfranchise minorities — a practice outlawed by the 1965 Voting Rights Act.

Is this a new strategy?
No. The word gerrymander comes from a salamander-shaped district in Massachusetts, redrawn that way in 1812 by then-Gov. Elbridge Gerry. But computerized voter-registration rolls have made the practice far more sophisticated, widespread, and effective. In the 2012 election, Republicans won 1.4 million fewer votes than Democrats, but still secured a dominant, 234-201 House majority. A frustrated President Obama raised the issue in his State of the Union address this year, calling for changes to a system that allowed "politicians [to] pick their voters, and not the other way round."

Why are districts redrawn?
The Constitution requires that states use new census data to redraw their electoral maps every 10 years, so that each district has roughly the same number of voters. In all but six states, where independent commissions draw the district lines, that process is run or overseen by whichever party controls the state legislature, with the governor able to veto redrawn maps. In the 2010 election cycle, the Republican Party took gerrymandering to a new level by creating a national redistricting strategy. The GOP's plan, which they called REDMAP, was to "keep or win Republican control of state legislatures with the largest impact on congressional redistricting."

How successful was it?
Overwhelmingly so. After pouring $30 million into state legislature races — a huge sum for down-ballot fights — the Republicans gained a record 680 state seats. That gave them total control of 25 state legislatures, which between them oversaw the redistricting process for 40 percent of congressional House of Representatives seats. Democrats were left with total redistricting control of just 10 percent of House seats; neither party has complete control over the redistricting process for the other 50 percent of seats. Using sophisticated mapping software (see below), Republican officials then redrew dozens of districts to their advantage. In 2012, these gerrymandered states helped the party protect its House majority. In Pennsylvania, House Republicans won 49 percent of the vote but took 72 percent of the seats (13 seats to the Democrats' five); in Ohio, they took 12 seats to four with only 52 percent of the vote.

What about the Democrats?
In states they control, they gerrymander just as aggressively. Democratic-controlled Maryland is one of the most gerrymandered states in the country; a U.S. Circuit Court judge once likened its 3rd Congressional District to a "broken-winged pterodactyl, lying prostrate across the center of the state." Illinois is another heavily gerrymandered blue state: Despite winning only 55 percent of the popular vote in 2012, Democratic House candidates captured 12 of the state's 18 seats. When President Obama was a state senator in Chicago, he helped redraw his district to include some of the city's wealthiest and most liberal residents, deepening the support base he later used to win his U.S. Senate seat. But Republicans have definitely gerrymandered more effectively — partly because geography already tilts the system in their favor.

In what way?
Democratic voters tend to be concentrated in compact urban areas. That means that even in a state with no gerrymandering, there would likely be a handful of very safe Democratic seats, with the rest of the state consisting of a larger collection of Republican-leaning rural and suburban districts. For this reason, many Republicans insist the effects of gerrymandering are exaggerated: When data analysts ran electoral scenarios that offset the effect of gerrymandering in 2012, they found that the Republicans still would have retained control of the House — albeit with a much smaller majority.

Is the system likely to change?
Several states are looking at handing responsibility for redistricting to an independent, nonpartisan commission, which has already been done in California, Arizona, and Washington. Computer software could easily redistrict maps in the fairest way possible, so that each district is a standard, geometric shape. But widespread change is unlikely. Politicians rarely relinquish their own power, and most attempts to overhaul the rules tend to result in lengthy court battles. For now and the foreseeable future, as GOP strategist and REDMAP architect Karl Rove put it, "he who controls redistricting can control Congress."

Making the map red
When gerrymandering districts, political cartographers are no longer limited to demographic data from the U.S. census. They can now purchase vast databases from companies like Facebook and Amazon, and find out exactly what people in a certain area are buying, or reading, or thinking. When this information is combined with previous election results and census data — on categories such as gender, race, and religion — it provides a highly reliable picture of how an area is most likely to vote. With advanced mapmaking software, it is then very easy for redistricting officials to create boundary lines that offer their party the maximum possible advantage. If they move a city block from one district to another, for example, it'll tell them the likely effect on all the surrounding districts. As journalist David Daley puts it in a forthcoming book on the subject: "The data and the technology make tilting a district map almost as easy as one-click ordering on Amazon."