Part one: Proto-prog and fullblown prog.

‘Progressive’ as a cultural phenomenon was first expressed in recorded popular music around 1964 via ‘progressive’ segments of songs (for instance, Animals, Beatles, Yardbirds, Who, Beach Boys) followed by entire progressive songs in 1966 (Soft Machine, Jimi Hendrix, Pink Floyd, Doors, Beatles) and entire progressive rock (a.k.a. prog) albums in 1968 (such as The Nice ‘Ars longa vita Brevis’, East Of Eden 'Mercator Projected', Moody Blues 'On the Threshold of a Dream', Caravan 'Caravan' (?), HP Lovecraft 'II' (?)). However, on the live scene Soft Machine and Pink Floyd were playing entire prog gigs as early as 1966.

In Denmark and other countries outside the UK and the US, progressive rock often developed at a slower pace.

In can be helpful to describe the development of progressive rock and any of it’s genres on a song and album basis by a stage model including research, ideation, conceptualization, realization, commercialization and operation. The three former stages define proto-prog, the latter fullblown prog.

On the song basis, Pink Floyd, Procol Harum, The Doors and others successfully reached the commercialization stage around 1967 while as an album genre, prog would not reach the realization stage before 1969’s ‘In the Court of the Crimson King’ by King Crimson and the commercialization stage a year later.

(Part 2 and subsequent parts deal with advances in technology and societal changes that supported the early development of progressive rock.)

Part two: The musical instruments, the instrumentalists and the producers.

Integration of musical instruments from jazz (brass instruments, the Hammond organ etc.), European classical music (string and woodwind instruments e.g. the clavichord and harpsichord but also the harmonium), Indian classical music (e.g. the sitar), electronic music (the Theremin, Clavioline, electroacoustic tape music, Musique Concrète, electronic generators etc.), blues and folk music (e.g. the 12-string guitar) into popular music in the 60s i.e. R&B, beat and rock, as well as the introduction of new (electronic) musical instruments such as the mellotron and moog synthesizer are prominent features in the development of progressive rock. (An example of instrument integration: the use of Harpsichord by Jimi Hendrix in 'Burning of the Midnight Lamp'.)

In addition, the use of the recording studio as a musical instrument (recall Joe Meek and Abbey Road Studios) plays an important role.

The producer would also become an important component of and have a significant effect on the music. In passing we mention Joe Meek, George Martin, Brian Wilson, Chris Blackwell, Teo Macero, Norman Smith and Johnny Reimar.

Also, the players or instrumentalists such as the keyboardist (Keith Emerson), the guitarist (Steve Howe) and the bassist (Chris Squire) would fill larger roles resulting in less lyrics (and perhaps melody) oriented music with increased focus on instrumental passages and the proficiency of the instrumentalist. By adding instrumentalists from genres outside popular music the group would become enlarged by musicians from e.g. jazz (e.g. Burnin Red Ivanhoe) and classical music (The Beatles).