In parallel voting, voters cast two (or more) votes, one for each method the system contains.^{[citation needed]} However, these votes do not interact in any way: the vote in one method has no effect on the calculation of seats in the other methods.

Under the most common form of parallel voting, a portion of seats in the legislature are filled by the single-member first-preference plurality method (FPP), while others are filled by proportional representation.^[3] This sometimes leads to a hypercorrection that attempts to limit the term parallel voting to refer only to mixtures of first-past-the-post and proportional representation. Parallel voting can use other systems besides FPP, and can have any mixture of winner-take-all, semi-proportional, and proportional components.

Although the two are often mistakenly conflated, mixed-member majoritarian representation and parallel voting refer to two different things. Parallel voting refers to a rule for computing each party's representation in a legislature, which involves two voting systems operating in parallel, with one being layered (superimposed) on top of the other. By contrast, mixed-member majoritarian representation refers to the results of the system, i.e. the system retains the advantage that some parties get in the winner-take-all side of the system.

For this reason, parallel voting is not always mixed-member majoritarian. For example, parallel voting may use a two proportional systems like STV and list-PR and then it would not be mixed-member majoritarian, and a majority bonus system (which is not the same as parallel voting) may also be considered mixed majoritarian. In addition, some mixed-member majoritarian systems are not parallel, in that they allow for interaction (limited compensation) between the two components, for example this is the case in South Korea and Mexico. In South Korea, the hybrid of parallel voting and seat linkage compensation, being between the MMP and MMM type of representation has been called mixed-member semi-proportional representation as well.^{[citation needed]}

Unlike mixed-member proportional representation, where party lists are used to achieve an overall proportional result in the legislature, under parallel voting, proportionality is confined only to the list seats. Therefore, a party that secured, say, 5% of the vote will have only 5% of the list seats, and not 5% of all the seats in the legislature.

The major critique of parallel systems is that they cannot guarantee overall proportionality. Large parties can win very large majorities, disproportionate to their percentage vote.

Parallel voting systems allow smaller parties that cannot win individual elections to secure at least some representation in the legislature; however, unlike in a proportional system they will have a substantially smaller delegation than their share of the total vote. This is seen by advocates of proportional systems to be better than elections using only first-past-the-post, but still unfair towards constituents of smaller parties. If there is also a threshold for list seats, parties which are too small to reach the threshold are unable to achieve any representation, unless they have a very strong base in certain constituencies to gain individual seats.

Smaller parties are still disadvantaged as the larger parties still predominate. Voters of smaller parties may tactically vote for candidates of larger parties to avoid wasting their constituency vote. If the smaller party close to the threshold may refrain from voting for their preferred party in favour of a larger party to avoid wasting their list vote as well. In countries where there is one dominant party and a divided opposition, the proportional seats may be essential for allowing an effective opposition.

Those who favour majoritarian systems argue that supplementary seats allocated proportionally increases the chances that no party will receive a majority in an assembly, leading to minority or coalition governments.^{[citation needed]}; the largest parties may need to rely on the support of smaller ones in order to form a government. Those who favour proportional representation see this as an advantage as parties may not govern alone, but have to compromise. It is also argued that parallel voting does not lead to the degree of fragmentation found in party systems under pure forms of proportional representation.^[4]

Because voters have two votes, one for a constituency candidate and one for a list, there is a critique that two classes of representatives will emerge under a parallel voting system: with one class beholden to their electorate seat, and the other concerned only with their party. Some consider this as an advantage as local as well as national interests will be represented. Some prefer systems where every constituency and therefore every constituent has only one representative, while others prefer a system where every MP represents the electorate as a whole as this is reflected in the electoral system as well.

Parallel systems are often contrasted with mixed-member proportional systems (MMP) or the additional member system (AMS). There are a unique set of advantages and disadvantages that apply to these specific comparisons.

A party that can gerrymander local districts can win more than its share of seats. So parallel systems need fair criteria to draw district boundaries. (Under MMP a gerrymander can help a local candidate, but it cannot raise a major party’s share of seats, while under AMS the effects of gerrymandering are reduced by the compensation)

Japan, and subsequently Thailand and Russia adopted a parallel system to provide incentives for greater party cohesiveness.^[5] The party is sure to elect the candidates at the top of its list, guaranteeing safe seats for the leadership. By contrast, under the MMP or AMS system a party that does well in the local seats will not need or receive any compensatory list seats, so the leadership might have to run in the local seats.

Certain types of AMS can be made de facto parallel systems by tactical voting and parties using decoy lists, which (other) MMP systems generally avoid. This specific type of tactical voting does not occur in parallel voting systems as there is no interaction between its systems to exploit in a way that makes it irrelevant. However, other types of tactical voting (such as compromising) are more relevant under parallel voting, than under AMS, and are virtually irrelevant under MMP.^{[citation needed]} Tactical voting by supporters of larger parties in favour of allied smaller parties close to a threshold, to help their entry to parliament are a possibility in any parallel, AMS or MMP system with an electoral threshold.

Parallel systems support the creation of single-party majorities more often than MMP or AMS systems, this may be a positive or a negative depending on the view of the voter.

Parallel voting is currently used in the following countries:^[6]

Country	Body	Members elected in constituencies			Members elected by proportional representation			Other members
		Total	%	System	Total	%	System	Total	System	%
Andorra	General Council	14	50%	PBV	14	50%	List PR
Japan	House of Representatives	289	62%	FPTP	176	38%	List PR
	House of Councillors	147	60%	SNTV	98	40%	List PR
Kazakhstan	Majilis	69	30%	FPTP	69	70%	List PR
Kyrgyzstan	Supreme Council	36	40%	FPTP	54	60%	List PR
Lithuania	Seimas	71	50%	TRS	70	50%	List PR (largest remainder method): open lists
Mongolia[7]	State Great Khural	78	62%	BPV	48	38%	List PR: closed lists
Nepal	House of Representatives	165	60%	FPTP	110	40%	List PR: closed lists
Philippines	House of Representatives	253	80%	FPTP	63	20%	List PR (Hare quota): closed lists
	Bangsamoro Parliament	32	40%	FPTP	40	50%	List PR	8		10%
Russian Federation	State Duma	225	50%	FPTP	225[8][9]	50%	List PR (Hare quota): closed lists
Senegal	National Assembly	105	64%	FPTP	60	36%	List PR (largest remainder method)
Taiwan (Republic of China)	Legislative Yuan	73	65%	FPTP	34	30%	List PR	6	SNTV for indigenous seats	5%
Tajikistan	Assembly of Representatives	41	65%	TRS	22	35%	List PR
Thailand	House of Representatives	400	80%	FPTP	100	20%	List PR

The Philippines' electoral system for Congress is an exceptional case. Political parties running for party-list seats are legally required to be completely separate from those running in constituency seats. Furthermore, political parties are capped at 3 seats (out of 20% of seats, or about 60 seats). As a result, the mixed-member system utilized in the Philippines is not representative at all of the share of the vote that "normal" political parties obtain (even amongst mixed-member majoritarian systems), let alone for those in full proportional representation systems.

Country	Body	Members elected in constituencies			Members elected by proportional representation
		Total	%	System	Total	%	System
South Ossetia	Parliament	17	50%	FPTP	17	50%	List PR

Country	Body	Members elected in constituencies			Members elected by proportional representation			Other members
		Total	%	System	Total	%	System	Total	System	%
Realm of New Zealand	Niue Assembly	14	70%	FPTP	6	30%	Plurality block voting (BV)
British overseas territories	Anguilla House of Assembly	7	54%	FPTP	4	31%	Plurality block voting (BV)	2	2 ex officio	15%
	Turks and Caicos Islands House of Assembly	10	48%	FPTP	5	24%	Plurality block voting (BV)	6	4 appointed, 2 ex officio	28%
	British Virgin Islands House of Assembly	9	60%	FPTP	4	27%	Plurality block voting (BV)	2	2 ex officio	13%

Country	Body	Members elected in constituencies			Members elected by proportional representation
		Total	%	System	Total	%	System
Argentina	Legislature of Córdoba Province	26	37%	FPTP	44	63%	List PR
	Legislature of Río Negro Province	24	52%	List PR	22	48%	List PR
	Chamber of Deputies of San Juan	19	53%	FPTP	17	47%	List PR
	Chamber of Deputies of Santa Cruz	14	58%	FPTP	10	42%	List PR

• Mexico's Chamber of Deputies uses a mixed-member majoritarian system for 300 first-past-the-post seats and 200 list PR (Hare quota) seats. It, however is not a parallel voting system since the two votes are fused and also, the two tiers are not completely independent of each other, these is conditional, partial seat linkage compensation. In contrast to the Chamber of Deputies, for electing the Chamber of Senators (upper house), a single (party list) vote is used similarly to the Italian system. However, constituencies have 3 seats with a type of limited (party block) voting being used: 2 seats are given to the largest party and 1 to the second largest party. Party-list PR is used for the nationwide seats.
• Hungary's National Assembly uses a system where the parallel voting component shares a pool of seats (93) with the vote transfer system and with the minority list seats with a reduced entry threshold. This means the number of seats effectively assigned proportionally based on the parallel party list votes is unknown/unknowable before the election takes place.^[10]
• Italy: Starting with the 2018 election, both houses of the Italian parliament are elected using a system similar to parallel voting. 62.5% of the seats are assigned proportionally to party lists; party lists are also linked in coalitions supporting constituency candidates running for the remaining 37.5% of the available seats, who are elected by means of a first-past-the-post system. Electors have a single vote with two-fold effects for a party list (proportional) and its associated local candidate (majoritarian). Split-ticket voting is not allowed, a voter may mark their ballots only next to a list, a candidate, or a list and a candidate associated with it and all of these votes has the same effect. If a voter marks a candidate not associated with the list they marked, like voters may under parallel voting, the vote is invalid under the Italian system.
• Jersey (UK)
• Monaco
• Pakistan
• Seychelles

• Albania used parallel voting in the 1996 and 1997 elections (before switching to mixed-member proportional representation from 2001 to 2005).^[11][12]
• Argentina: Santiago del Estero Province (1997–2009)
• Armenia
• Azerbaijan's National Assembly (the Milli Məclis) had previously used an SM system in which 100 members were elected for five-year terms in single-seat constituencies and 25 were members were elected by proportional representation. Since the latest election Azerbaijan has returned to electing members from single-member constituencies. Due to the corruption present within Azerbaijan,^[13] the limited proportionality that SM was able to offer had little effect.
• Bulgaria (1990, 2009)
• Croatia (1993–2001)
• Egypt (2020)
• Georgia (1990–2024): Georgia initially used a two-round system for its constituency seats. Up until 2016, 73 seats out of 150 seats were allocated in constituencies. In the 2020 election, this number was reduced to 30 out of 150 as a result of the 2019 protests. By 2024, Georgia will switch to a fully proportional electoral system.^[14]
• Italy (1993–2005, with modifications)
• North Macedonia (1998)
• Palestinian Authority (2005), for the next election, the system was changed to party-list proportional representation.
• South Korea: (1988-2024) National Assembly used parallel voting from 1988 to 2019. From 2019 to 2024, it uses a hybrid system of parallel voting and mixed-member proportional, with both compensatory seats (30) and supplementary seats (17).
• Ukraine: In the last elections to the Verkhovna Rada, a parallel voting system was used. 50% of seats are distributed under party lists with a 5% election threshold and 50% through first-past-the-post in single-member constituencies. The method of 50/50 mixed elections was used in the 2002, 2012, 2014 and 2019 elections; however, in 2006 and 2007, the elections were held under a proportional system only. According to the election law that became valid on 1 January 2020 the next election to the Verkhovna Rada again will be held under a proportional scheme.

In New Zealand, the Royal Commission on the Electoral System reviewed the electoral system in 1985–86 and considered parallel voting as a possible replacement for the single-member plurality (SMP) system in use at the time.

The commission came to the conclusion that parallel voting would be unable to overcome the shortcomings of New Zealand's previous SMP system. The total seats won by a party would likely remain out of proportion to its share of votes—there would be a "considerable imbalance between share of the votes and share of the total seats"—and it would be unfair to minor parties (who would struggle to win constituency seats).^[15] In the indicative 1992 electoral referendum, parallel voting was one of four choices for an alternative electoral system (alongside MMP, AV and STV), but came last with only 5.5 percent of the vote. An overwhelming majority of voters supported MMP, as recommended by the Royal Commission, and the system was adopted after the 1993 electoral referendum.

In another referendum in 2011, 57.77% of voters elected to keep current the MMP system. Among the 42.23% that voted to change to another system, a plurality (46.66%) preferred a return to the pre-1994 SMP system. Parallel voting was the second-most popular choice, with 24.14% of the vote.^{[citation needed]}