Semi-closed primaries are a type of electoral system used in some U.S. states during the selection of candidates for the general election. In this system, registered members of a political party can vote in their party's primary, while unaffiliated voters can choose to participate in either party's primary. This allows for greater voter participation while still maintaining some level of party control. This system contrasts with open primaries, where any registered voter can participate regardless of party affiliation, and closed primaries, where only registered party members can vote. Semi-closed primaries aim to balance inclusivity and party integrity, impacting the nomination process for candidates like presidential candidates and congressional candidates.