quote from wiki
In 1960, Gibson Les Paul sales were significantly lower than they had been in previous years, so in 1961 the model was given a completely new body style that was thinner and had 2 sharp cutaway horns that made the upper frets more accessible. The neck was slightly heavy, which made it tilt downwards. The neck joint was also moved up about 3 frets. It was felt the new design could compete with the popular Fender Stratocaster, another benefit being lower production costs than that of the previous model. The guitar was advertised as having the "fastest neck in the world", due to its slender neck profile and virtually non-existent heel. The newly designed Les Paul was popular but Les Paul, whose namesake was carried over from the previous version, did not like the new design and asked to have his name removed from it. His reasons were simple: He thought the new neck joint was not as stable as the previous models, and didn't want his name on something that could break. Gibson renamed the model the "SG" which was short for "solid guitar". Even though Les Paul's name was officially removed from the model in 1961, the plastic Les Paul nameplates (positioned between the rhythm pickup and fingerboard) were in abundance in the Gibson factory and SG models having these nameplates were built and sold by Gibson up to the end of 1963.
