An LP rotates at a steady 33 ⅓ revolutions per minute. Where did that number come from? The answer dates back almost a hundred years ago, and it has as much to do with movies as it does with music.
Recorded sound began with Thomas Edison and his wax cylinder in 1877. By the 1890s, folks were hustling to improve on his concept, and inventor Emile Berliner had the innovative idea to replace the cylinder with a flat disc. Thus the first "records" were born. But in order to play them, someone had to manually crank the handle of a Victrola to rotate the platter. As you can imagine, this led to inconsistent playback speeds (and tired arms). Eventually, a motor was added to ensure steady rotation. At the time, two of the most common components available were 3,600 RPM motors and gears with a 46:1 ratio, and this combination resulted in a speed of 78 RPMs. 78 became the standard speed through the 1920s, although 78-RPM discs could only fit about 5 minutes of audio on each side.
In 1931, RCA Victor introduced the first 33 ⅓-RPM records and players, but they were a commercial failure. As it turns out, the height of the Great Depression was not the ideal time to launch an expensive & novel consumer technology. But Hollywood exerted its influence in a different way. The advent of "talkies" in the 1920s created a need for longer sound recordings. A standard 1,000-foot roll of 35-millimeter film played for 11 minutes, so 78s were impractical to use as soundtracks. Enter the Vitaphone system in 1929, which used the aforementioned 3,600-RPM motor but switched to a gear with a 108:1 ratio, resulting in a speed of 33 ⅓ RPM and contained a full reel's worth of audio.
Although sound-on-disc for movies was replaced by "optical soundtracks" in the early 1930s, the notion of a musical recording that lasted longer than the five-minute 78 appealed to radio programmers. In the early 1940s, Columbia Records pioneered "microgroove" technology, which squeezed a groove's width down to a millimeter and allowed 20 minutes of music to fit onto one side of a 12-inch, 33 ⅓-RPM record. The LP ("long-playing") format was introduced in 1948, and it has been the industry standard for vinyl albums ever since.
So now you know!