District lines are redrawn on a rhythm most voters never see. The cycle is anchored to the census, and the intervals within it determine how much of a decade any map governs.
Population counts start the clock
The census counts residents once every ten years, and those counts determine how many congressional seats each state receives through apportionment.
Detailed population data for small geographies is released afterward, and states cannot draw compliant maps until they have it.
That sequence compresses the drawing process into the period between data delivery and the first election filing deadlines that follow.
Equal population is the governing constraint
Districts within a state must contain nearly equal populations, a requirement derived from constitutional interpretation rather than from statute.
Because people move, districts drift out of balance over a decade, and redrawing restores equality using the new count.
Congressional districts are held to a tighter standard of equality than state legislative districts, which are permitted somewhat more variance.
Who draws the map varies by state
In many states the legislature draws districts and passes them like ordinary legislation, subject to the governor's veto where one applies.
Others assign the task to commissions, which differ in whether members are independent of the parties, appointed by them, or a mixture.
A few states use backup mechanisms that transfer the task to a court or panel if the primary body fails to produce a map in time.
Litigation extends the process past adoption
Adopted maps are frequently challenged under federal voting rights provisions or state constitutional requirements concerning compactness and county integrity.
Because cases take time, elections are sometimes conducted under a map that is still being litigated, and a later ruling can require redrawing mid-decade.
Courts weigh the disruption of changing rules close to an election, which is why timing arguments feature heavily in redistricting cases.
The cycle shapes political incentives
Whoever controls the drawing process in a given year influences the map for the rest of the decade, which makes the elections immediately before a census unusually consequential.
Map drawers work with detailed data on past voting behavior and can evaluate proposals against expected outcomes before adopting them.
Reform proposals generally target either who draws the lines or what criteria bind them, and states have adopted both approaches in different combinations.