Redistricting, which determines how many representatives come from each district in the nation, is completed every 10 years. The process occurs after the U.S. Census is taken to calculate how many people live in each city and county.
Prior to a series of decisions from the U.S. Supreme Court in the 1960s, states completed the redistricting process on their own schedule, and they often ignored the process for decades. This led to situations where small districts had the same amount of representation as large districts, which meant that some residents were underrepresented. Each legislative body usually draws its own district lines.