It's possibly one of the most denigrated inventions in the history of music; the greatest signifier of rock star pomposity. Indeed, in some quarters, the very mention of it is likely to provoke sniggering derision, conjuring up images of quadruple-gatefold album sleeves, songs that go on for weeks and straggly-haired rockers prattling on about mystical lands, unicorns, goblins and dystopian futures. But - back when people actually took the time to sit down and listen to records from beginning to end - for many, nothing delivered a more rewarding experience than the concept album. And for some, it's still a format that provides rock music with its high watermark moments.