The current system for creating stereophonic (2 channel) phonograph discs date back to 1933 when Alan Blumlein, a senior sound engineer at EMI in London, demonstrated a single-groove system in which the stylus moves both horizontally and vertically.
When the 12-inch Long Play record was launched in 1948, it was initially monophonic, and it wasn’t until 1957 that stereophonic LPs were released, by now using a refined version of the EMI system developed by Westrex (a division of Western Electric) called Westrex 45/45 in which each channel drives the cutting head at a 45 degree angle to the vertical.
In late 1957, Audio Fidelity Records and Bel Canto in the US released demonstration stereo LPs, with the the Bel Canto release on multicoloured vinyl. The first mass-produced stereophonic LPs were released in early 1958.
Mono LPs continued to be released alongside stereo LPs for the next ten years or so with major labels ceasing production in 1968, but 7-inch singles continued to be mono for longer, into the 1970s in some cases.
Stereo records produced using the Westrex system played well on a mono record player, and mono records could be played on a stereo system.
In the 1970s, quadraphonic (4 channel) LPs were produced, but were not a great success partly because there were several competing and incompatible systems.