Its name suggests it was invented by a wizard, and it takes a genius to play one. The Continuum is one of the most fantastically named musical instruments, a keyboard without keys that online encyclopedia Wikipedia claims only two people can play.
One of them is Jordan Rudess, a member of US progressive metal act Dream Theater, who are performing in Hong Kong for the first time next week.
'I wanted to create my own instrument around the idea of a fretless keyboard,' Rudess says on the phone from Hiroshima. 'We were bouncing around the idea of something where you could travel around from note to note. For years I'd planned on making it myself, and then we discovered some fellow had made something called a Continuum in Canada.
'So I went up and checked it out and I was really blown away, thinking, 'I just got to get me one of these'. So I bought one.'
If that sounds like a typically rock-star statement, that's because Rudess is a rock star with one of the biggest, longest-surviving bands to never enter the mainstream consciousness. The most recent member to join the colossus that is Dream Theater seven years ago, Rudess has since mastered the Continuum and in doing so has helped redefine the band's sound.
Perhaps just as difficult as playing a keyboard without keys is finding a satisfying, one-size-fits-all definition of progressive rock.
Songs are divided into various parts or movements, much like classical music. Lengthy solos for each instrument and the evocation of science fiction, fantasy, religion and war are also familiar themes. In short, prog rock couldn't be more unfashionable if it tried.