"Duophonic" was used as a trade name for the process by Capitol Records for re-releases of mono recordings from June 1961 through the 1970s. Capitol employed this technique in order to increase their inventory of stereo LPs, to satisfy retailer demand for more stereo content (and help promote the sale of stereo receivers and turntables). For nearly ten years, Capitol used the banner "DUOPHONIC-For Stereo Phonographs Only" to differentiate their true stereo LPs from the Duophonic LPs.
The process was used for some of their biggest releases, including a variety of albums by The Beach Boys and Frank Sinatra. Over the years however, some Duophonic tapes were confused with true stereo recordings in Capitol Records' vaults, and wound up getting accidentally reissued on CD throughout the 1980s and 1990s. In recent years, Capitol reissued some of The Beatles' Duophonic mixes on The Capitol Albums, Volume 1 and The Capitol Albums, Volume 2, in 2004 and 2006, respectively.
Other record companies used similar processing of monophonic material to create a stereo effect, but referred to the process by other names. For example, Columbia Records used the logo, "Electronically Re-channeled for Stereo" on records issued with their particular process. As with Capitol, Columbia's fake stereo issues included albums by major artists, such as Miles Davis ('Round About Midnight-CL 949 mono, reissued in stereo as PC 8649).