 These trends peaked in the 1969 Woodstock festival, which saw performances by most of the major psychedelic acts.Progressive rock, a term sometimes used interchangeably with art rock, moved beyond established musical formulas by experimenting with different instruments, song types, and forms.From the mid-1960s the Left Banke, the Beatles, Queen, the Rolling Stones and the Beach Boys, had pioneered the inclusion of harpsichords, wind, and string sections on their recordings to produce a form of Baroque rock and can be heard in singles like Procol Harum's "A Whiter Shade of Pale" (1967), with its Bach-inspired introduction.