Stereo is the reproduction of sound using two separate audio channels in a way that creates the feeling of sound heard from spacious directions, as in natural hearing.
Stereophonic technology was developed in the 1930s by Alan Blumlein at EMI. After an evening at the cinema, Blumlein was frustrated that the sound on screen could only be heard from one speaker. To solve this, the engineer began working on a binaural system which is still in use now.
Incidentally, Blumlein was killed in a plane crash in the World War, and he never reduced the stereo system to actual practice. EMI cut the first stereo test discs using the system in 1933, but it was not applied commercially until a quarter of a century later.
In the '60s, it was prevalent practice to generate stereo editions of music from monophonic originals, which were usually labeled 'reprocessed' or 'enhanced' stereo.
The influence of changing the audio format is felt even today, in countless reissues of mono records, including The Beatles entire discography.
Bruce Spizer, a Beatles expert said:
“When the early Beatles albums were recorded in 1963 through 1967, mono was by far the dominant format in the UK. Beatles producer George Martin recorded these albums with mono in mind from the start. The stereo mixes were an afterthought, with the Beatles rarely present for the stereo mixing sessions. For these reasons, the early mono albums represent the Beatles music as the Beatles and George Martin intended it be heard.”
Here is a sample of the modern take on natural mono sound: