"Once a decade after the census, every state redraws the districts used to elect members of the U.S. House of Representatives and state legislators - a process known as redistricting. When drawing these lines, there are some requirements in federal law that all states must follow. For example, all states must ensure that districts have approximately the same number of people and comply with the Voting Rights Act." - Brennan Center
The statewide proportion of districts whose voters, based on statewide state and federal partisan general election results during the last ten years, favor each political party shall correspond closely to the statewide preferences of the voters. (NCSL)
For example, when electing a five-person body from a region inhabited by people whose viewpoints are 55% A, 20% B, 20% C and 5% D, a proportional body would consist of three A representatives, one B representative, and one C representative (see graphic).
Prohibition on favoring or disfavoring an incumbent, candidate or party. The prohibition in a given state may be broader, covering any person or group, or it may be limited to intentionally or unduly favoring a person or group. Details on these prohibitions are included in the state descriptions below. (NCSL)
The degree to which minorities have the opportunity to elect representatives of their choice.
"Don't draw lines that set out to harm voters based on their race or ethnicity. Where discrimination plays or has played a significant role, and where voting is substantially polarized along racial or ethnic lines, look at electoral patterns and decide whether minorities already have proportionate electoral power." - LLS
District shapes should be compact. Having the minimum distance between all the parts of a constituency (a circle, square or a hexagon is the most compact district).
"No contorted boundaries. A district with smoother boundaries will be more compact, and one with more squiggly boundaries will be less compact.
Less dispersion. A district with few pieces sticking out from the center will be more compact, and one with pieces sticking out farther from the district's center will be less compact." - LLS
This map is an example of what is not compact. See how it is spread out in a bizarre shape.
Maintain districts as previously drawn, to the extent possible. This leads to continuity of representation. (NCSL)
A "community of interest" is a group of people with a common interest. For example "social, cultural, racial, ethnic, and economic interests common to the population of the area, which are probable subjects of legislation. Several states consider keeping "communities of interest" whole when drawing state legislative and congressional districts. - LLS
All parts of a district being connected at some point with the rest of the district. (NCSL)
Contiguity is the most common rule imposed by most states. A district is contiguous if you can travel from any point in the district to any other point in the district without crossing the district's boundary. Put differently, all portions of the district are physically adjacent.
The prohibition in a given state may be broader, covering any person or group, or it may be limited to intentionally or unduly favoring a person or group. (NCSL)
The Supreme Court has unanimously stated that excessive partisanship in the process is unconstitutional, but the Court has also said that federal courts cannot hear claims of undue partisanship because of an inability to decide how much is "too much." (LLS)
The U.S. Constitution requires that each district have about the same population: each federal district within a state must have about the same number of people, each state district within a state must have about the same number of people, and each local district within its jurisdiction must have about the same number of people.
States must make a good-faith effort to draw districts with the same number of people in each district within the state, and any district with more or fewer people than the average must be justified by a consistent state policy. - LLS