I remember when I saw my first double-neck guitar, I was really impressed. That seemed like a huge thing to have hanging from your neck. They became really popular in the Led Zeppelin, Yes and The Who era. The idea is to have 2 guitars handy, like a 6-string and a 12-string, or a bass and a guitar, a fretted and a fretless bass, or 2 guitars in a different tuning. But obviously it’s also one of those macho ‘size-matters’ things. The guitarist with the double-neck here is Jimmy Page from Led Zeppelin.
There’s more, of course: e.g. Steve Vai has played the triple-neck heart-shaped monster you see on the side.
Today, triple neck guitars are still rare, because they’re huge, heavy, expensive and utterly pointless. They’re an obscene symbol of self-indulgence, like Missy Elliot’s Lamborghini bed, or Snoop’s jewel-encrusted crunk cup.
King of the hill: the quintuple-neck guitar, used by Cheap Trick’s Rick Nielsen. That is: 36 strings (4 guitars and a 12 string). Respect!