the major elements of the psychedelic sound to audiences in this period, such as guitar feedback, the Indian sitar and backmasking sound effects. Psychedelic rock particularly took off in California's emerging music scene as groups followed the Byrds's shift from folk to folk rock from 1965. The psychedelic lifestyle, which revolved around hallucinogenic drugs, had already developed in San Francisco and particularly prominent products of the scene were Big Brother and the Holding Company, the Grateful Dead and Jefferson Airplane. The Jimi Hendrix Experience's lead guitarist, Jimi Hendrix did extended distorted, feedback-filled jams which became a key feature of psychedelia. Psychedelic rock reached its apogee in the last years of the decade. 1967 saw the Beatles release their definitive psychedelic statement in Sgt. Pepper's Lonely Hearts Club Band, including the controversial track "Lucy in the Sky with Diamonds", the Rolling Stones responded later that year with Their Satanic Majesties Request, and Pink Floyd debuted with The Piper at the Gates of Dawn. Key recordings included Jefferson Airplane's Surrealistic Pillow and the Doors' Strange Days. These trends peaked in the 1969 Woodstock festival, which saw performances by most of the major psychedelic acts. Sgt. Pepper was later regarded as the greatest album of all time and a starting point for the album era, during which rock music transitioned from the singles format to albums and achieved cultural legitimacy in the mainstream. Led by the Beatles in the mid-1960s, rock musicians advanced the LP as the dominant form of recorded music expression and consumption, initiating a rock-informed album era in the music industry for the next several decades. Progressive rock 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, 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. The Moody Blues used a full orchestra on their album Days of Future Passed (1967) and subsequently created orchestral sounds with synthesizers. Classical orchestration, keyboards, and synthesizers were a frequent addition to the established rock format of guitars, bass, and drums in subsequent progressive rock. Instrumentals were common, while songs with lyrics were sometimes