A nonpartisan blanket primary is a primary election in which all candidates for the same elected office run against each other at once, regardless of the political party. Partisan elections are, on the other hand, segregated by political party. Nonpartisan blanket primaries are slightly different from most other elections systems with two-rounds/runoff, aka "jungle primaries" (such as the Louisiana primary), in a few ways. The first round of a nonpartisan blanket primary is officially the "primary." Round two is the "general election." Round two must be held, even if one candidate receives a majority in the first round.

In addition, there is no separate party nomination process for candidates before the first round. Also, political parties are not allowed to whittle down the field using their internal techniques (such as party primaries or conventions). It is entirely possible that multiple candidates of the same political party advance to the general election.

In most cases, two winners advance to the general election, in which case it is also called a top-two primary. If more than two candidates are selected for the general election, it may be known as a top-four primary or top-five primary. It is also known as a jungle primary.

This system theoretically elects more moderate candidates, since winning might require appealing to voters of both parties in a two-party system. However, all primaries use plurality voting and are susceptible to vote-splitting: the more candidates from the same party that run in the primary, the more likely that party is to lose. Research on California's primaries have shown no increase in moderate candidates, and no increase in turnout among nonpartisan voters. Some have proposed using other voting systems in the primary to alleviate this problem, such as the unified primary based on approval voting for its first round.

The top-two system is used for all primaries in Washington and California except presidential primaries, and Alaska began using a top-four primary system in the 2022 Alaska's at-large congressional district special election using ranked-choice voting.

The so-called Louisiana primary is similar. With a first round to pick the top two candidates and a second-round to choose between these two. The differences are that the first election is the general election. The second is a later "runoff" election, and there is no second round if a candidate wins more than half the votes in the first round.