Rotosound is a British guitar and bass string manufacturing company based in England.
[1][2] Rotosound's most famous string set, the RS66 Swing Bass, was first produced in 1966.
John Entwistle of The Who came to the Rotosound factory looking for an even-sounding, heavy, roundwound bass string.
[1][3] A fake jingle for Rotosound can be heard on The Who's 1967 album The Who Sell Out, immediately preceding "I Can See for Miles".
Jaco Pastorius was also a dedicated user of Swing 66 bass strings.