The b-side was officially born in 1902, when the International
Zonophone Company developed a method for producing double-sided record
discs. Previously, music had been recorded using either single-sided discs or the metal-coated (and later,
celluloid)
cylinders popularized by
Thomas Edison's gramophone models. By 1904, double-sided discs were on the market -- mostly recordings of
operas and
orchestral music. As the record length was limited to four minutes per side (at
78 rpm), b-sides were usually continuations of the a-side recording.
After the size of the discs was increased to 10- and 12-inch sizes in order lengthen playtimes (and to allow b-side tracks to appear in earnest), discs grew in popularity. The double-sided disc format was truly popularized by the German Odeon label, which published the first "album" set in 1909, Tchaikovsky's Nutcracker Suite, in a four-disc set. It was about this time that technology caught up to the practicality of a disc player, and the use of cylinders waned.
As record players became more common, popular music became more accessible. In the 1920's, commercial radio stations began to pop up across the world, including several which broadcasted strictly recorded music. Also, the invention and proliferation of jukeboxes in the late 1920s and 1930s brought recorded music to new (and younger) audiences.
Record labels wanted information about which singles were receiving the greatest play in the jukeboxes and sales in the stores. At this time, there still wasn't much distinction placed between a-sides and b-sides. Billboard first compiled these stats in July 1940, giving radio stations a list of the top singles to play, and labels a forum in which to compete with each other. Suddenly, recording sessions concentrated on getting a 'hit' record, an often the poor b-side was a throwaway track.
The b-side assumed its most commonly-recognized form in 1947 when RCA Victor introduced the 7-inch 45 rpm extended play vinyl record (commonly known as the 45). The 45 single became the format of choice for DJs and jukeboxes and, in conjunction with the introduction of the 33 1/3 rpm long playing (LP) record, signaled the demise of the 78. The cheaper 45 also proved popular with American and British teenagers, who fueled the rapid growth of a new musical genre: rock and roll.
The 1950s and early 1960s were the heyday for the 45 single (and thus the b-side). And while a-side tracks usually received more care and attention than b-sides, some exceptions can be observed. On occasion, a b-side track wound up being received much better than its counterpart. Examples of this phenomenon are: Richie Valens' "La Bamba" (b-side to "Donna"), The Guess Who's "Undun" (b-side to "Laughing"), David Bowie's "Velvet Goldmine" (b-side to "Space Oddity"), The Smiths' "How Soon Is Now" (b-side to "William It Was Really Nothing") and Madonna's "Get Into The Groove" (b-side to "Angel"). On other occasions, singles climbed the charts as a-side/b-side pairs, like Creedence Clearwater Revival's "Who'll Stop The Rain"/"Travelin' Band" did in 1967.
The 1970s signaled yet another change in the music industry. Many rock artists (most notably Led Zeppelin and Pink Floyd) moved towards album-oriented rock, eschewing the idea of releasing singles. R&B and disco artists had no such aversion, however, and the a-b format only gained ground in these areas. Another setback to the traditional b-side appeared at this time as well: magnetic tape. With eight track tapes, and later cassettes, the concept of a b-side became more nebulous. Portable tape players and smaller radio sets helped to contribute to the weakening of the demand for jukeboxes, further eroding the grip of the b-side.
In the 1980s, fewer artists were releasing singles, as Billboard and other music charts began basing their singles charts on radio airplay as opposed to sales or jukebox plays. With the introduction of the compact disc in 1982, the definition of the b-side began to change. Instead of being filler material on the flipside of a 45 or LP, a b-side began to be defined either as filler material that may appear anywhere on an album, or that was not included on an album .
Lately, artists have been issuing compilations of b-side material. Some noteworthy examples include:
Adam Ant - B-Side Babies (1994)
Sarah McLachlan - Rarities, B-Sides and Other Stuff (1995)
U2 - Best of 1980-1990/B-Sides (1998)
Beck - Stray Blues: A Collection of B-Sides (2000)
Pixies - Complete B-Sides (2001)
The Charlatans UK - Songs From The Other Side (2002)
Sources:
Recording Technology History --
http://history.acusd.edu/gen/recording/notes.html
BBC History of Vinyl --
http://www.bbc.co.uk/aboutmusic/features/vinyl/
The All Music Guide --
http://www.allmusic.com