Information:  - Procol Harum are an English rock band formed in 1967. They contributed to the development of symphonic rock, and by extension, progressive rock. Their best-known recording is their 1967 hit single "A Whiter Shade of Pale", which is considered a classic of popular music and is one of the few singles to have sold over 10 million copies. Although noted for its baroque and classical influence, Procol Harum's music also embraces the blues, R&B, and soul.  - A power trio is a rock and roll band format having a lineup of guitar, bass and drums, leaving out the second guitar or keyboard that are used in other rock music quartets and quintets to fill out the sound with chords. While one or more band members typically sing, power trios emphasize instrumental performance and overall impact over vocals and lyrics. History. The rise of the power trio in the 1960s was made possible in part by developments in amplifier technology that greatly enhanced the volume of the electric guitar and bass. Particularly, the popularization of the electric bass guitar defined the bottom end and filled in the gaps. Since the amplified bass could also now be louder, the rest of the band could also play at higher volumes, without fear of being unable to hear the bass. This allowed a three-person band to have the same sonic impact as a large band but left far more room for improvisation and creativity, unencumbered by the need for detailed arrangements. As with the organ trio, a 1960s-era soul jazz group centered on the amplified Hammond organ, a three-piece group could fill a large bar or club with a big sound for a much lower price than a large rock and roll band. A power trio, at least in its blues rock incarnation, is also generally held to have developed out of Chicago-style blues bands such as Muddy Waters' trio.  - Robin Leonard Trower (born 9 March 1945) is an English rock guitarist and vocalist who achieved success with Procol Harum during the 1960s, and then again as the bandleader of his own power trio.  - 20th Century Blues is the fifteenth studio album by Robin Trower .    What is the relationship between '20th century blues ' and 'blues rock'?
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