The 1952 Constitution of the Commonwealth of Puerto Rico, under which the Caribbean island organized as a self-governing polity associated to the United States, established a republican form of government with legislative, judicial and executive branches. Legislative power is vested in a Legislative Assembly, composed of a 27-seat Senate and a 51-seat House of Representatives. Members of the Legislative Assembly are directly elected every four years by universal adult suffrage, under an electoral system that provides for three types of representation in both the Senate and the House: district-level, at-large, and additional or minority party representation.

For the purposes of district-level representation, Puerto Rico is divided in eight Senate districts of approximately equal population, each one of which elects two Senators, for a total of sixteen district Senators. Voters may choose up to two candidates, and may split their two votes among candidates from different parties. The two candidates with the largest number of votes in each district are elected to the Senate. Usually the two district Senators belong to the same party, but split-ticket voting makes it possible for candidates from opposing parties to be elected at the same time. Nonetheless, since the establishment of the Puerto Rican Senate in 1917, split-outcome district Senate races have been exceptional as well as recent, with two cases in 1992, one in 1996, one in 2004, one in 2016 and two in 2020.

Each Senate district is divided into five House districts of approximately equal population, each one of which elects one Representative by plurality voting, for a total of forty district Representatives.

In both the Senate and the House, a total of eleven members are elected on an at-large basis. Under this procedure, parties may nominate up to eleven candidates in each house, but voters may choose only one at-large Senate candidate, and one at-large House candidate. At-large candidates run as individuals rather than as a list, but parties determine the ballot order of their at-large candidates, which varies across Puerto Rico's election precincts in order to insure each candidate has an approximately equal chance of being elected. A voter casting a straight-ticket ballot automatically chooses the at-large candidates placed at the top of his or her party's Senate and House lists; nonetheless, voters may choose any single at-large candidate from any party or independent ticket (or even vote for a write-in candidate) for each legislative body.

It should be noted that the distribution of at-large seats is determined to a degree by the number of candidates nominated by each party: there have been instances in which minor parties have failed to win a single at-large seat in either house because their vote was divided among too many candidates, as well as cases in which parties have lost the opportunity to secure additional at-large seats because they nominated too few candidates. In practice, Puerto Rico's two major parties - the Popular Democratic Party (PPD) and the New Progressive Party (PNP) - nominate six candidates for each house, while the Puerto Rican Independence Party (PIP), which has a much smaller following, fields single candidates for both the Senate and the House.

It should also be noted that - unlike in general elections - in PNP and PPD primary elections, voters may choose up to six at-large candidates for each legislative body.

At-large voting as implemented in Puerto Rico is known internationally as the Single Non-Transferable Vote (SNTV) system, although this term is almost never used in the island. Incidentally, the non-transferability of the vote refers to the fact that voters in Puerto Rico do not indicate transferable second, third, fourth and successive preferences, as they would do under the Single Transferable Vote (STV) system used in parliamentary elections in Ireland and Malta, as well as the Australian Senate. On the other hand, the procedure used to select at-large legislative candidates in party primaries is known as the Multiple Non-Transferable Vote (MNTV) or block voting system.

Finally, the Constitution of Puerto Rico provides for an increase in the membership of the Senate or the House of Representatives (or both bodies), in the event the majority party obtains over two-thirds of the seats in one or other of the two houses, in order to insure adequate representation to minority parties. However, there is a cap of nine minority seats in the Senate and seventeen in the House - one-third of the original membership in each legislative body. When the majority party polls less than two-thirds of the vote for Governor of Puerto Rico, minority seats in the Senate or the House are distributed by proportional representation according to the minority parties' vote for Governor; a similar but different procedure is provided for in the event the majority party wins more than two-thirds of the vote for Governor, but it has never been invoked. In either case, additional seats assigned to a minority party go to defeated at-large candidates with the largest vote totals, and then to district candidates with the largest proportion of the vote who have not been elected.

Minority parties must obtain at least three percent of the vote for Governor in order to participate in the distribution of additional legislative seats. It should also be noted that the constitutional provisions regarding minority party legislative representation do not take into account potential differences between the results of the gubernatorial election and the legislative races. This was not a problem in 1952, when nearly one hundred percent of voters cast straight ballots and election results were almost identical for all elective offices, but by 1972 it had become a controversial issue. In that year's election - which had a then-record number of split-ticket votes - the Popular Democratic Party captured more than two-thirds of the seats in both the Senate and the House of Representatives, and the Puerto Rican Independence Party received two minority seats in the House. Meanwhile, former Governor Roberto Sánchez-Vilella won 59,855 votes (4.6%) as an at-large House candidate for the People's Party (PP; a short-lived PPD breakaway), but PP won only 4,007 votes (0.3%) for Governor and was therefore excluded the distribution of additional seats; the second PIP seat went to Luis Angel Torres, who had polled 127 at-large votes. The controversy wound up in court, which eventually confirmed Torres' election.

Although at-large representation and additional representation help minority parties in Puerto Rico, the distribution of seats in both houses of the Legislative Assembly is largely determined by the plurality voting-based district races, and the winning party often attains representation well above its percentage of the vote in both bodies, at the expense of minority parties.