33/LP Records...

The 12-inch 33-rpm vinyl disc was introduced in 1948 by CBS Records and could play from 25 to 30 minutes per side. It revolved at 33 and 1/3 revolutions per minute and could hold the average symphony, sonata, or quartet on a single side, thus alleviating the problem found with 78's, which broke up the continuity of moving parts. By 1950, a pattern was set: 12-inch LPs for popular albums, 45s for individual popular songs. The 33 was produced in either a monophonic sound format, or a stereo sound format, and by the late 1960's, most American record companies had discontinued their monaural records, except for the release of historical issues. This new record format, along with the 45-rpm, led to the 78-rpm phonograph record having an early retirement and vanishing into oblivion. (Click here for the source.)