In the late 1960s, acid rock had reached the pinnacle of its popularity. Born from the hippie counterculture, acid rock was named due to its use as background music for drug-induced trips--especially on LSD, which is also known as "acid." There were popular underground parties that were hosted primarily in San Francisco that would feature performances from acid rock bands such as The Grateful Dead.
The musical structure of acid rock often involves repetitive themes with verses and choruses, as well as long, improvised instrumental solos. The instrumentation almost always included a line-up of vocals, one or more electric guitars, bass guitar and drums, but it wasn't restricted to that combination. Keyboards, most specifically the electric organ, were also largely used, and were often one of the primary solo instruments. The guitars were often used with distortion, phaser and other effects to bring the music to another level of experimentation. Vocals were used sparingly in comparison to other popular songs of the time, and were most typically used exclusively through verses and choruses.
There were many bands who created acid rock music, however none may have been as influential as The Doors, who were referred to as the "Kings of Acid Rock" in a Time article written by Fred Prowledge. Jimi Hendrix is known as one of the greatest guitarists of all time. His band, The Jimi Hendrix Experience, was another of the most influential bands in acid rock. In addition to those two bands, the following are also considered as champions of acid rock: Jefferson Airplane, Cream, Iron Butterfly, the Grateful Dead, Dow Jones and the Industrials and early Pink Floyd.
As the 1970s aged, so did acid rock. Most of the larger bands involved with the movement had broken up or shifted their musical direction. Acid rock in itself had evolved. Bands like Alice Cooper and Deep Purple, who in the early-to-mid 70s were identified as releasing acid rock music, were now being referred to as "heavy metal." In addition to heavy metal, many hard rock groups cite bands like the Doors, Jefferson Airplane and Cream as major influences to their music.
Music today continues to be influenced by many characteristics of acid rock. The Dave Matthews Band's extended rock jams draw on influences from the Grateful Dead and the long jams that are typical of the genre. The harder, progressive nature of The Doors and Iron Butterfly can be heard in music from bands like Rush and Coheed and Cambria. Even in music that is seemingly unrelated, the experimental nature of acid rock paved the way for many different sounds and effects to be used in all kinds of music.