The process of dividing a fixed number of legislative seats among political groupings (states, counties, or districts) based on population. At the federal level, the 435 U.S. House seats are reapportioned among states after each decennial census. Redistricting then draws the internal boundaries within each state's allotted seats.

A constitutionally mandated count of every person living in the United States, conducted every ten years. Census results determine apportionment of House seats, drive redistricting, and inform how federal funding is allocated. The Census Bureau releases block-level population data that map drawers use as the raw input for new district lines.

The smallest unit of geography used by the Census Bureau. Census blocks are bounded by streets, water features, and legal boundaries. They are the building blocks that map drawers aggregate into precincts, then into districts.

A collection of contiguous census blocks, typically containing 600 to 3,000 people. Block groups are a common unit of analysis for demographic data and are often used in redistricting software to build district populations.

A geographic area whose residents share cultural, historical, economic, or social interests, who benefit from being represented together. Most state redistricting criteria require or encourage map drawers to keep communities of interest whole, rather than splitting them across multiple districts. COIs are distinct from partisan affiliation: a community of interest could be a farming region, a university town, a coastal fishing area, or an immigrant enclave.

A geometric measure of how "normal" a district's shape looks. Compact districts tend to be roughly circular or square; non-compact ones are elongated, jagged, or have many arms. Common mathematical measures include the Polsby-Popper ratio (area vs. perimeter) and the Reock score (area vs. minimum bounding circle). Courts and state laws often require districts to be "reasonably compact," though there is no single legal definition.

A district where every part is physically connected to every other part, with no gaps or isolated pockets. Contiguity is a near-universal requirement in redistricting law. Districts can be contiguous but still highly non-compact, connected only by a narrow corridor.

A gerrymandering technique where a politically cohesive group is deliberately split across multiple districts so that the group is a minority in each one, diluting its collective electoral influence. Cracking is the opposite of packing.

A defined geographic boundary within which residents elect a single representative to a legislative body. Districts exist at multiple levels: U.S. Congressional districts (one per House seat per state), state senate districts, state house districts, city council districts, and more. Every address in the U.S. falls within multiple overlapping districts simultaneously.

The once-per-decade redrawing of district boundaries that follows the Census. All U.S. House seats and most state legislative seats must be redrawn after each census to reflect population shifts. The last two redistricting cycles were 2011 (based on the 2010 census) and 2021 (based on the 2020 census).

A mathematical measure of partisan bias in a district map. It calculates the difference in "wasted votes" between two parties across all districts in a plan. A wasted vote is any vote cast for a losing candidate, plus any vote cast for the winning candidate beyond what was needed to win. An efficiency gap of 0 means both parties waste the same proportion of votes. A gap above roughly 8% is considered evidence of significant partisan advantage.

The practice of drawing district boundaries to advantage a particular political party, candidate, or demographic group. Named after Massachusetts Governor Elbridge Gerry, whose 1812 district was said to resemble a salamander. Modern gerrymandering is typically accomplished through packing and cracking. It is distinguished from "political" map-drawing by intent: a gerrymander deliberately distorts representation, rather than simply reflecting natural population patterns.

An open geographic data format based on JSON that encodes geographic features (points, lines, polygons) alongside their properties. GeoJSON is the most common format for sharing district boundary files, precinct maps, and community-of-interest submissions. VoteValue uses GeoJSON for district rendering and the map editor uses the VVR format (a zip of GeoJSON + metadata).

A redistricting body whose members are selected outside of the legislature, designed to reduce the conflict of interest that arises when legislators draw the districts they will run in. The selection process varies: some states use citizen applicant pools, others use panels of retired judges or party-balanced appointees. States with independent commissions include California, Arizona, Michigan, Colorado, and Virginia. The independence and effectiveness of these bodies varies significantly in practice.

A district in which a racial or ethnic minority group constitutes a majority of the voting-age population. The Voting Rights Act of 1965 requires that map drawers not dilute minority voting strength, and courts have sometimes required the creation of majority-minority districts to provide minority communities with a realistic opportunity to elect representatives of their choice.

The constitutional requirement, established by the Supreme Court in Reynolds v. Sims (1964) and Wesberry v. Sanders (1964), that legislative districts must have substantially equal populations so that each voter's ballot carries approximately the same weight. Congressional districts must be almost exactly equal in population. State legislative districts are held to a looser standard: deviations of up to 10% are generally permissible if justified by legitimate state interests.

A gerrymandering technique where a politically cohesive group is concentrated into as few districts as possible, so that the group "wastes" many votes on massive supermajority wins while having little influence elsewhere. Packing is the opposite of cracking.

A systematic advantage that a district map gives to one political party relative to the other, measured by comparing how seat shares translate from vote shares. A map with partisan bias produces different seat outcomes depending on which party earns a given share of votes: for example, consistently translating 50% of votes into 60% of seats for one party.

A compactness metric equal to 4π × (area / perimeter²). It ranges from 0 to 1, where 1 is a perfect circle. Lower scores indicate more jagged, elongated, or irregular district shapes. It is the most commonly cited compactness measure in court cases and academic research on redistricting.

A measure of how closely a district map translates statewide vote shares into seat shares. A perfectly proportional plan would give a party that wins 55% of votes roughly 55% of seats. In practice, geographic clustering of voters means some deviation is inevitable, but large deviations signal partisan manipulation. Dave's Redistricting App scores every map on proportionality (0–100) as one of its five primary rating dimensions, alongside competitiveness, minority representation, compactness, and county splitting.

The smallest administrative unit for conducting elections. Precincts are defined by state and local governments and determine where voters go to cast ballots. Precinct-level election returns (who voted for whom in each precinct) are the fundamental data used to measure competitiveness, detect packing and cracking, and calculate the efficiency gap. VoteValue uses precinct-level VEST data to compute VVI scores.

The process of redrawing the geographic boundaries of legislative districts. Redistricting occurs at the federal, state, and local levels after each decennial census. Who draws the maps varies by state: in some, the state legislature draws them; in others, an independent or bipartisan commission does. The process determines which voters are grouped together for representation, directly shaping electoral outcomes for the next decade.

A government body tasked with drawing or recommending district maps, as an alternative to the state legislature drawing its own maps. Commissions vary widely: some are independent (members chosen outside the legislature), some are bipartisan (equal party representation), and some are advisory (the legislature can still override). States with independent redistricting commissions include California, Arizona, Michigan, and Colorado.

The rules that map drawers must follow when creating district lines. Federal requirements include equal population (one person, one vote) and compliance with the Voting Rights Act. Most states add additional criteria such as contiguity, compactness, preservation of political subdivisions, communities of interest, and (in some states) partisan fairness. The order of priority matters: some criteria can override others when they conflict.

A compactness metric equal to a district's area divided by the area of the smallest circle that can completely enclose the district. It ranges from 0 to 1, where 1 is a perfect circle. Unlike Polsby-Popper, the Reock score is less sensitive to perimeter irregularities and more sensitive to the overall elongation of a shape. VoteValue uses both as part of the District Integrity sub-score.

The degree to which a map divides counties, cities, or towns across multiple districts. Most state redistricting criteria require preserving political subdivisions where possible, because splitting a county or city can fragment local community interests and make representation harder to navigate. Dave's Redistricting App rates every map on a "least splitting" dimension (0–100), measuring how well the map keeps counties and municipalities whole.

A district where neither party has a dominant structural advantage and the outcome regularly shifts between parties based on candidate quality, turnout, and political environment. Swing districts are the primary target of both voter mobilization campaigns and campaign spending. VoteValue's Competitiveness sub-score (35% of VVI) directly measures how "swing" a district is by looking at recent election margins, partisan volatility, and historical flip history.

VoteValue's proprietary composite score (0–100) that measures the structural electoral power of a single vote in a given district. It combines five sub-scores: Competitiveness (35%), Mobilization Potential (20%), District Integrity (20%), Electoral Leverage (15%), and Race Significance (10%). Higher scores mean your individual ballot has more structural impact on outcomes in that district. Scores are calculated at the precinct level and are specific to each address.

A landmark federal law enacted in 1965 that prohibits voting practices that discriminate on the basis of race, color, or membership in a language minority group. In redistricting, Section 2 of the VRA prohibits map drawers from diluting minority voting strength; for example, by cracking minority communities across multiple districts or packing them into too few. Compliance with the VRA is a mandatory criterion in all redistricting, ranking above state-law criteria when they conflict.

The count of people who are legally eligible to vote in a given area, as distinct from the total population or the registered voter count. VEP excludes non-citizens and (depending on state law) people with certain felony convictions. VoteValue uses VEP data alongside actual turnout to calculate the Mobilization Potential sub-score: a measure of how many eligible voters didn't participate and could potentially change future outcomes.

Votes that do not contribute to electing a candidate. There are two kinds: votes cast for a losing candidate (all wasted), and votes cast for the winning candidate beyond the minimum threshold needed to win (surplus votes, also wasted). The efficiency gap compares wasted votes between parties across all districts in a plan. Gerrymandering typically works by engineering one party's losses to waste as many of the opposing party's votes as possible.