From The Art and Popular Culture Encyclopedia
The 12-inch [30 cm] single gramophone record came into existence with the advent of disco music in the 1970s. The first 12" single was actually a 10" acetate used by a mix engineer (Jose Rodriquez) in need of a Friday night test copy for famed disco mixer Tom Moulton. As no 7" acetates could be found, a 10" blank was used. Mr. Moulton, feeling silly with a large disc that only had a few centimeters of groove on it, asked Rodriguez to re-cut it so the grooves looked more spread out. Because of the wider spacing of the grooves, a broader overall dynamic range (distinction between loud and soft) was made possible. This was immediately noticed to give a more favorable sound for discotheque play.
The first official promotional 12" single was Southshore Commission's "Free Man". At first, these special versions were only available as promotional copies to DJs. By 1976, with the release of "Ten Percent" by Double Exposure, the new format was sold to the general public. But also "Theme From Shaft" by Isaac Hayes (Stax 5C 052Z-62266 released 1971) could be one of the first.