Fair districting is drawing district lines in a way that does not favor political parties or incumbents. Whether it is possible to achieve fair districting depends on the dimension of fairness one is assessing. It may be difficult or even impossible to achieve fairness along all lines.

On August 6, 1965, President Johnson signed The Voting Rights Act of 1965 ("the Act"), federal legislation that prohibits racial discrimination in voting. The Act defines certain elements of fair districting from a legal standpoint. Section 2 of the Act forbids deprivation of voting rights for failure to pass a literacy test as well as "vote dilution," under which a person's voting power or usefulness is reduced.¹ Fair districting requires the absence of vote dilution.

There are multiple mechanisms through which vote dilution can occur. Vote dilution most commonly occurs when district lines are drawn in a way to push minority communities into a small number of districts (packing) or distribute them widely into a large number of districts (cracking, fracturing, or splitting).² An at-large election can also dilute the votes of minorities in that all voters are permitted to vote for all seats. This process allows majority voters to override minority interests by voting for representatives consistent with their beliefs.³

To prove that a redistricting plan has diluted minority votes, the Supreme Court has ruled that the following three conditions be met:

1. A minority "is sufficiently numerous and compact to form a majority in a single-member district;"
2. A minority is "politically cohesive" (meaning its members tend to vote similarly); and
3. The "majority votes sufficiently as a bloc to enable it ... usually to defeat the minority's preferred candidate."⁴