The 12-inch single came into existence with the advent of disco music in the 1970s. The benefits of using 12″ singles was discovered using a 10″ acetate. Mix engineer Jose Rodriquez was in need of a Friday night test copy for famed disco mixer Tom Moulton. But he had no 7″ (18 cm) acetates available so he used a 10″ (25 cm) blank instead.
Moulton, feeling silly with a large disc which only had a couple of inches of groove on it, asked Rodriguez to re-cut it so that 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 discothèque play.
Moulton’s position as the premiere mixer and “fix it man” for pop singles ensured that this fortunate accident would instantly become industry practice. This would perhaps have been a natural evolution: As songs became much longer than had been the average for a pop song, and the DJ in the club wanted sufficient dynamic range, the format would have surely had to be changed from the 7 inch (18 cm) single eventually.
Also worth noting is that the visual spacing of the grooves on the 12″ assisted the DJ in locating the approximate area of the “breaks” on the disc’s surface (without having to listen as he dropped and re-dropped the stylus to find the right point). A quick study of any DJ’s favorite discs will reveal mild wear in the “break points” on the discs surface that can clearly be seen by the naked eye, which further eases the “cueing” task (a club DJ’s tone-arm cartridge will be heavily weighted and mild wear will seldom spoil the sound quality). Many DJ-only remix services, such as Ultimix and Hot Tracks, issued sets with deliberately visualised groove separations (i.e., the record was cut with narrow and wider spacings that could be seen on the surface, marking the mix points on the often multi-song discs).
A broader dynamic range or louder recording level requires more space as the grooves’ excursions (i.e., the width of the groove waves and distance traveled from side to side by the turntable stylus) become much greater in amplitude, especially in the bass frequencies so important for dance music. Many record companies began producing 12-inch (30 cm) singles at 33⅓ rpm, as the slower speed enhances the bass on the record. By the same token, however, 45 rpm gives better treble response and was used on many 12-inch singles, especially in the UK.