The terms A-side and B-side refer to the two sides of 78, 45, and 33​1⁄3 rpm phonograph records, or cassettes, whether singles, extended plays (EPs), or long-playing (LP) records. The A-side usually featured the recording that the artist, record producer, or the record company intended to receive the initial promotional effort and then receive radio airplay, hopefully, to become a "hit" record. The B-side (or "flip-side") is a secondary recording that has a history of its own: some artists released B-sides that were considered as strong as the A-side and became hits in their own right. Others took the opposite approach: producer Phil Spector was in the habit of filling B-sides with on-the-spot instrumentals that no one would confuse with the A-side. With this practice, Spector was assured that airplay was focused on the side he wanted to be the hit side.

Music recordings have moved away from records onto other formats such as CDs and digital downloads, which do not have "sides", but the terms are still used to describe the type of content, with B-side sometimes standing for "bonus" track.^[1]

The first sound recordings at the end of the 19th century were made on cylinder records, which had a single round surface capable of holding approximately two minutes of sound. Early shellac disc records records only had recordings on one side of the disc, with a similar capacity (both media could hold between three and four minutes by 1910). Double-sided recordings, with one selection on each side, were introduced in Europe by Columbia Records in 1908, and by 1910 most record labels had adopted the format in both Europe and the United States; the ability to effectively double the amount of sound on the disc was one major factor in its rising to dominance over the cylinder record which was obsolete by 1912.

There were no record charts until the 1930s, and radio stations (by and large) did not play recorded music until the 1950s (when top 40 radio overtook full-service network radio). In this time, A-sides and B-sides existed, but neither side was considered more important; the "side" did not convey anything about the content of the record.

In June 1948, Columbia Records introduced the modern 33​1⁄3 rpm long-playing (LP) microgroove vinyl record for commercial sales, and its rival RCA Victor, responded the next year with the seven-inch 45 rpm vinylite record, which would quickly replace the 78 for single record releases. The term "single" came into popular use with the advent of vinyl records in the early 1950s. At first, most record labels would randomly assign which song would be an A-side and which would be a B-side. (All records have specific identifiers for each side in addition to the catalog number for the record itself; the "A" side would typically be assigned a sequentially lower number.) Under this random system, many artists had so-called "double-sided hits", where both songs on a record made one of the national sales charts (in Billboard, Cashbox, or other magazines), or would be featured on jukeboxes in public places.

As time wore on, however, the convention for assigning songs to sides of the record changed. By the early sixties, the song on the A-side was the song that the record company wanted radio stations to play, as 45 rpm single records (or '45s') dominated the market in terms of cash sales. It was not until 1968, for example, that the total production of albums on a unit basis finally surpassed that of singles in the United Kingdom.^[2] In the late 1960s, stereo versions of pop and rock songs began to regularly appear on 45s. The majority of the 45s were played on AM radio stations, which were not equipped for stereo broadcast at the time, so stereo was not a priority. However, the FM rock stations did not like to play monaural content, so the record companies adopted a protocol for DJ versions with the mono version of the song on one side, and stereo version of the same song on the other.

By the early 1970s, double-sided hits had become rare. Album sales had increased, and B-sides had become the side of the record where non-album, non-radio-friendly, instrumental versions or simply inferior recordings were placed. In order to further ensure that radio stations played the side that the record companies had chosen, it was common for the promotional copies (DJ version) of a single to have the "plug side" on both sides of the disc.

With the decline of 45 rpm vinyl records, after the introduction of cassette and compact disc singles in the late 1980s, the A-side/B-side differentiation became much less meaningful. At first, cassette singles would often have one song on each side of the cassette, matching the arrangement of vinyl records, but eventually, cassette maxi-singles, containing more than two songs, became more popular. Cassette singles were phased out beginning in the late 1990s, and the A-side/B-side dichotomy became virtually extinct, as the remaining dominant medium, the compact disc, lacked an equivalent physical distinction. However, the term "B-side" is still used to refer to the "bonus" tracks or "coupling" tracks on a CD single.

With the advent of downloading music via the Internet, sales of CD singles and other physical media have declined, and the term "B-side" is now less commonly used. Songs that were not part of an artist's collection of albums are made available through the same downloadable catalogs as tracks from their albums, and are usually referred to as "unreleased", "bonus", "non-album", "rare", "outtakes" or "exclusive" tracks, the latter in the case of a song being available solely from a certain provider of music.

B-side songs may be released on the same record as a single to provide extra "value for money". There are several types of material commonly released in this way, including a different version (e.g., instrumental, a cappella, live, acoustic, remixed version or in another language), or, in a concept record, a song that does not fit into the story line.

Additionally, it was common in the 1960s and 1970s for longer songs, especially by soul, funk, and R&B acts, to be broken into two parts for single release. Examples of this include Ray Charles's "What'd I Say", the Isley Brothers' "Shout", and a number of records by James Brown, including "Papa's Got a Brand New Bag" and "Say It Loud - I'm Black and I'm Proud". Typically, "part one" would be the chart hit, while "part two" would be a continuation of the same performance. A notable example of a non-R&B hit with two parts was the single release of Don McLean's "American Pie". With the advent of the 12in single in the late 1970s, the part one/part two method of recording was largely abandoned. Modern day examples are Fall Out Boy's EP, My Heart Will Always Be The B-Side To My Tongue, or My Chemical Romance's The Black Parade, The B-Sides

Since both sides of a single received equal royalties, some composers deliberately arranged for their songs to be used as the B-sides of singles by popular artists. This became known as the "flipside racket". Similarly, it has also been alleged that owners of pirate radio stations operating off the British coast in the 1960s would buy the publishing rights to the B-sides of records they expected to be hits, and then plug the A-sides in the hope of driving up sales and increasing their share of the royalties.

Occasionally, the B-side of a single would become the more popular song. This sometimes occurred because a DJ preferred the B-side to its A-side and played it instead. Some examples include "I Will Survive" by Gloria Gaynor (originally the B-side of "Substitute"), "Ice Ice Baby" by Vanilla Ice (originally the B-side of "Play That Funky Music"), "I'll Be Around" by the Spinners (originally the B-side of "How Could I Let You Get Away") and "Maggie May" by Rod Stewart (originally the B-side of "Reason to Believe"). Probably the most well-known of these, however, is "Rock Around the Clock" by Bill Haley & His Comets (originally the B-side of "Thirteen Women (And Only One Man In Town))".

The song "How Soon Is Now?" by the Smiths started out as the extra track on the 12-inch of William, It Was Really Nothing but later gained a separate release as an A-side in its own right, as did Oasis's "Acquiesce", which originally appeared as a B-side to "Some Might Say" in 1995, but gained subsequent release in 2006 as part of an EP to promote their forthcoming compilation album, Stop the Clocks. Feeder in 2001 and 2005 had the B-sides "Just a Day" from "Seven Days in the Sun", and "Shatter" from "Tumble and Fall" released as A-sides after fan petitions and official website and fansite message board hype, and both charted at No. 12 and No. 11 in the UK. In 1986, the first single from XTC's record Skylarking, "Grass", was eclipsed in the United States by its B-side, "Dear God" – so much so that the record was almost immediately re-released with one song ("Mermaid Smiled") removed and "Dear God" put in its place, becoming one of the band's better-known hits.

On many reissued singles, the A- and B-sides are two hit songs from different albums that were not originally released together, or were by completely different artists, altogether. These were often made for the jukebox, as one record with two popular songs on it would make more money, or to promote an artist to the fans of another. For example, in 1981 Kraftwerk released their new single "Computer Love" coupled with the B-side "The Model", from their 1978 LP The Man-Machine. With synthpop increasingly dominating the UK charts, the single was re-released with the sides reversed. In early 1982 "The Model" reached number one.

A "double A-side" is a single where both sides are designated the A-side; there is no B-side on such a single. In 1949, Savoy Records promoted a new single by one of its artists, Paul Williams' "House Rocker" and "He Knows How to Hucklebuck", as "The New Double Side Hit – Both Sides "A" Sides".^[3] In 1965, Billboard reported that due to a disagreement between EMI and John Lennon about which side of the Beatles' "We Can Work It Out" and "Day Tripper" single should be considered the A-side and receive the plugging, "EMI settled for a double-side promotion campaign—unique in Britain."^[4] They continued to use the format for the release of the singles "Eleanor Rigby" and "Yellow Submarine" in 1966, followed by "Strawberry Fields Forever" / "Penny Lane" in 1967, and "Something" / "Come Together" in 1969. Other groups followed suit, notably the Rolling Stones in early 1967 with "Let's Spend the Night Together" / "Ruby Tuesday" as a double-A single.

A double A-sided single is often confused with a single where both sides, the A and the B, became hits. Although many artists in the late 1950s and early 1960s like Elvis Presley, the Everly Brothers, Fats Domino, Ricky Nelson, the Beach Boys, Brenda Lee, and Pat Boone, routinely had hit singles where both sides of the 45 received airplay, these were not double A-sides. The charts below tally the instances for artists' singles where both sides were hits, not where both sides were designated an A-side upon manufacture and release. For instance "Don't Be Cruel", the B-side of "Hound Dog" by Elvis Presley, became as big a hit as its A-side even though "Don't Be Cruel" was not the intended A-side when released in 1956. Reissues later in the 1960s (and after the Beatles' "Day Tripper"/"We Can Work It Out") listed the single with both songs as the A-side. Also, for Cliff Richard's 1962 "The Next Time"/"Bachelor Boy", both sides were marketed as songs with chart potential, albeit with "Bachelor Boy" pressed as the B-side.

In the UK, before the advent of digital downloads, both A-sides were accredited with the same chart position, as the singles' chart was compiled entirely from physical sales. In the UK, the biggest-selling non-charity single of all time was a double A-side, Wings' 1977 release "Mull of Kintyre"/"Girls' School", which sold over two million copies. It was also the UK Christmas No. 1 that year, one of only two occasions on which a double A-side has topped that chart, the other being Queen's 1991 re-release of "Bohemian Rhapsody" with "These Are the Days of Our Lives".^[5] Nirvana released "All Apologies" and "Rape Me" as a double A-side in 1993, and both songs are accredited as a hit on both the UK Singles Chart,^[6] and the Irish Singles Chart.^[7]

Queen released their first double-A single, "Killer Queen"/"Flick of the Wrist", in 1974. "Killer Queen" became a hit, while "Flick of the Wrist" was all but ignored for lack of promotion. Three years later, they released "We Are the Champions" with "We Will Rock You" as a B-side. Both sides of the single received much radio airplay (often one after the other), which led to them sometimes being referred to as a double A-side. In 1978 they released "Fat Bottomed Girls"/"Bicycle Race" as a double A-side; that time both sides of the single became hits.

Occasionally double-A-sided singles were released with each side targeting a different market. During the late 1970s, for example, Dolly Parton released a number of double-A-sided singles, in which one side was released to pop radio, and the other side to country, including "Two Doors Down"/"It's All Wrong, But It's All Right" and "Baby I'm Burning"/"I Really Got the Feeling". In 1978, the Bee Gees also used this method when they released "Too Much Heaven" for the pop market and the flip side, "Rest Your Love on Me", which was aimed toward country stations.

Many artists continue to release double A-sided singles outside of the US where it is seen as more popular. Examples of this include Oasis's "Little by Little"/"She Is Love" (2002), Bloc Party's "So Here We Are"/"Positive Tension" (2005) and Gorillaz's "El Mañana"/"Kids with Guns" (2006).

Artists having the most US double-sided singles on which each side charted in the US Hot 100, according to Billboard:^[8]

Artists having the most US double-sided singles on which each side reached the Billboard Top 40, according to Billboard:^[8]

On vinyl, double A-sided singles had one song on either side of the record, while double B-sides contained two songs on the same side (on the B-side, making three songs in all). When such singles were introduced in the 1970s, the popular term for them was "maxi single", though this term is now used more ambiguously for a variety of formats. For some people these records would not quite qualify as EPs, for those generally have four songs on a 45.

Genesis's 1978 7-inch single "Many Too Many" featured two B-sides, "The Day the Light Went Out" and "Vancouver", both of them being outtakes from the ...And Then There Were Three... album. There was no 12-inch equivalent. The band released two 7-inch singles with three tracks apiece, Spot the Pigeon and 3X3 (also known as "Paperlate"), which were explicitly marked as EPs. "Spot the Pigeon" was also available in a 12-inch version, and also subverted this format a bit, by having two tracks on the A-side and one track on the B-side. The B-side, "Inside and Out", was also considered the selling point of the EP, being Steve Hackett's last contribution to the band, and remains a favorite of many fans.

Paul McCartney's 1980 single "Coming Up" had a studio version of the song on the A-side, while the B-side contained two songs, a live version of "Coming Up" and a studio instrumental called "Lunchbox/Odd Sox".

Iron Maiden's 1980 7-inch single "Sanctuary" was a re-recording of a song that had been given for use on the Metal For Muthas compilation the previous year. The recording was made during the Iron Maiden sessions but was left off the UK version of that album, and was then put out as a single. To help compensate fans who had specifically bought Metal for Muthas for the track, the "Sanctuary" single had two live B-sides which were deliberately selected to be non-album tracks—"I've Got The Fire" (a cover of the Montrose song) and "Drifter". A studio recording of "Drifter" (featuring Adrian Smith instead of Dennis Stratton) appeared on their next album, Killers, and a studio version of "I've Got The Fire" featuring Bruce Dickinson appeared on the B-side of "Flight of Icarus" a few years later. At the time this single was released they were the first live Iron Maiden tracks released (though more would follow), and it remains the only officially released recording of "I've Got The Fire" with Paul Di'Anno on vocals.

The singles from U2's album The Joshua Tree were released with two B-side songs each, which were pressed at 33​1⁄3 rpm. Versions for jukeboxes included only one of those songs, which played at 45 rpm.

The UK 7-inch single of "Love Shack" by The B-52's was released with live versions of "Planet Claire" and "Rock Lobster" on the B-side, which plays at 33​1⁄3 rpm. The follow-up "Roam" followed suit, including live versions of "Whammy Kiss" and "Dance This Mess Around" on the B-side playing at 33​1⁄3 rpm.

The Rolling Stones released "Brown Sugar" from their album Sticky Fingers in May 1971. While the American single featured only "Bitch" as the B-side, the British single added a third track, a live rendition of "Let It Rock" (the Chuck Berry classic) recorded at the University of Leeds during their 1971 tour of the UK.

The concept of the B-side has become so well known that many performers have released parody versions, including:

The term "b/w", an abbreviation of "backed with", is often used in listings to indicate the B-side of a record. The term "c/w", for "coupled with", is used similarly.^[11]