The basis for these changes every 10 years is because of the U.S. census which takes place every 10 years in a year with a 0 at the end, like 1990, 2000, 2010. Reapportionment is the reassigning of representatives seats in the U.S. House of Representatives to the states through a complicated mathematical formula that adjusts the amount of seats assigned to each state based on their population. Redistricting is the the division of boundaries geographically, in order to determine who is represented and who is eligible to vote for whom on local, state and federal levels.