The 45-rpm record was developed by RCA Victor in 1949 immediately following the invention of "vinyl" plastic and the development of the 12" LP record. Initially, 45-rpm singles were able to play up to eight minutes per side, but by the mid 50's it was reduced to roughly 2 to 4 minutes and became a big hit with jukebox manufactures. It is normally 7 inches in diameter, although some 45 singles can be found on 12-inch diameter records, but still played at 45 RPM's. 45 sales peaked from 1973 to 1976 with more than 100 million being sold each year in the USA. Also, 45s made since 1971 are in stereo and have the same fidelity output as CDs (i.e. 20 Hz-20,000 Hz), and are still made today, with more than 2 million pressed in 1998! One interesting fact is that 78-33=45! Click here for the source.