Congressional districts within a state must contain nearly equal populations, and maps must comply with federal voting-rights and equal-protection rules. States choose legislatures, commissions, or other lawful institutions to draw lines, and may add criteria such as compactness, contiguity, or respect for communities.

Every map has political consequences because it groups voters, but consequence alone does not reveal an unlawful purpose. Analysts compare population equality, racial composition, partisan data, geographic criteria, procedural history, and alternative maps to identify what drove the line-drawing choices.

Redistricting changes constituencies as well as electoral prospects. A member may inherit new communities, lose familiar supporters, or face another incumbent. Those effects can alter representation and campaign strategy before the first vote is cast under the new map.

Video lesson: Gerrymandering: Crash Course Government and Politics #37