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Electoral concept in which a party receives "additional" seats due to quirks of the electoral system

Overhang seats are constituency seats won in an election under the traditional mixed member proportional (MMP) system (as it originated in Germany), when a party's share of the nationwide votes would entitle it to fewer seats than the number of individual constituencies won. The electoral reform in Germany removed the overhang seats, and replaced with Zweitstimmendeckung ("second vote coverage").

Under MMP, a party is entitled to a number of seats based on its share of the total vote. If a party's share entitles it to ten seats and its candidates win seven constituencies, it will be awarded three list seats, bringing it up to its required number. This only works, however, if the party's seat entitlement is not less than the number of constituencies it has won. If, for example, a party is entitled to five seats, but wins six constituencies, the sixth constituency seat is referred to as an overhang seat. Overhang can result from an unproportional distribution of constituencies as well as strong region based support or the existence of regional parties.

Overhang seats are dealt with in different ways by different systems.

A party is allowed to keep any overhang seats it wins, and the corresponding number of list seats allocated to other parties is eliminated to maintain the number of assembly seats. This means that a party with overhang seats has more seats than its entitlement, and other parties have fewer. This approach is used in the Chamber of Deputies of Bolivia and the National Assembly of Lesotho. It was unsuccessfully recommended by the 2006 Ontario Citizens' Assembly on Electoral Reform for adoption by the Legislative Assembly of Ontario, and the proposed Dual member proportional representation system uses this approach as well. While for the first additional list seats are simply denied to parties, in the latter three cases, a fairer procedure was proposed of subtracting the constituency seats won by parties with overhang seats from the total number of seats and recalculating the quota (the largest remainder method was also recommended) to proportionally redistribute the list seats to the other parties. In the Scottish Parliament, Welsh Senedd and London Assembly, the effect is similar, but the mechanism different. In the allocation of second vote list seats using a highest averages method, the first vote constituency seats already won are taken into account when calculating party averages, with the aim of making the overall result proportional. If a party wins more seats in the first vote than its proportion of second votes would suggest it should win overall, or if an independent candidate wins a constituency seat, then the automatic effect is to reduce the overall number of seats won by other parties below what they might expect proportionally.

A party is allowed to keep any overhang seats it wins, but other parties are still awarded the same number of seats that they are entitled to. This means that a party with overhang seats has more seats than its entitlement. The New Zealand Parliament uses this system; one extra seat was added in the 2005 election and 2011 election, and two extra seats in the 2008 election. This system was also used in the German Bundestag until 2013, when Germany switched to a fully compensatory system.

Other parties may be given additional list seats (sometimes called "balance seats" or leveling seats) lest they be disadvantaged. This preserves the same ratio between parties as was established in the election. It also increases the size of the legislature, as overhang seats are added, and there may also be extra list seats added to counteract them. This system results is less proportional than full compensation, as the party with the overhang is still receiving a "bonus" above its proportional entitlement. After the Federal Constitutional Court of Germany ruled in 2008 that the established implementation of Option 1 was unconstitutional (because in rare instances, it allowed votes to negatively affect the number of seats for a given party, contradicting the voter's will see below), full compensation of overhang seats is applied as of 2013 in the German Bundestag by providing for the first time for the creation of balancing seats. However, as the implementation of the system to equalize overhang seats led to a drastic increase of the size of the Bundestag, there have been ongoing reform efforts ever since.

A party is not allowed to keep any overhang seats it wins, with its number of seats actually being reduced until it fits the party's entitlement. This method raises the question of which constituency seats the party is not allowed to keep. After that is determined, it would then have to be decided who, if any, will represent these constituencies. This was used in the Landtag of Bavaria until 1966, with the candidates with the lowest number of votes not keeping the constituency seat. The Bavarian system allows for this more easily as both the first and second vote are "personalized" as they are both given to individual candidates who represent their party, the first vote going to a candidate standing in that District while the second is given to an individual candidate standing in the same region excluding those candidates standing in the same district. Unlike for federal and most other German state elections, in Bavarian state elections, the number of seats allocated in the Landtag is based on the combined share of both the first and second votes. Which individual candidates are entitled to the seats their party is allocated is again calculated based on the number of votes they received and not on their position on a predetermined closed list (as in federal elections). This is similar to panachage, a system used for local elections in Bavaria, that allows voters to alter the order of candidates on party lists with their general election vote. As a consequence of this system in the 1954 Bavarian state election two overhang seats were not awarded.

In state and federal elections in Germany, every constituency seat is granted while the electoral system requires that a party needs 5% of the party list vote to win list seats. Unless in federal elections a party wins at least three constituency seats, it is excluded by (i. e., due to) the election threshold. In 1994 the PDS fell below the 5% threshold but still got its share of list seats as it had won enough constituency seats, and the same happened in 2021 for Die Linke. But in 2002 the PDS won only two constituencies and was excluded from the list seats resulting in two overhang seats. The PDS had won four seats in Berlin at the 1998 election (in addition to surpassing the electoral threshold at 5.1% of the second vote) but partly due to redistricting, which changed the boundaries of constituencies the PDS had won in 1998, lost two of their constituency seats, including placing third behind Hans Christian Ströbele in the newly created Berlin Friedrichshain Kreuzberg – Prenzlauer Berg East constituency. In the 2021 federal election, The Left fell just short of the election threshold with 4.9% of the national vote but won three single member constituency seats; Berlin Lichtenberg, Berlin Treptow Köpenick and Leipzig II. This entitled them to proportional representation in the Bundestag according to their second votes. The extra compensation seats for other parties meant that the Bundestag would be the largest in German history, with 736 MPs.

In New Zealand, Te Pāti Māori (the Māori Party) usually gets less than 5% of the party votes, the threshold required to enter parliament – unless the party wins an electorate seat. Te Pāti Māori won one overhang seat in 2005 and 2011, and two overhang seats in 2008. In 2005 their share of the party vote was under 2% on the initial election night count, but was 2.12% in the final count which included special votes cast outside the electorate. On election night it appeared that the party, whose candidates had won four electorate seats, would get two overhang seats in Parliament. However, with their party vote above 2% the party got an extra seat and hence needed only one overhang seat. National got one less list seat in the final count, so then conceded defeat (the result was close between the two largest parties, National and Labour). Te Pāti Māori won six electorate seats in 2023, two more than its 3.08% share of the party vote entitled it to; and shortly after the general election, the Port Waikato by election resulted in a further overhang seat for National.