Postmodernism in music follows closely to its defining characteristics of the music itself rather than being a genre. It’s a defining period where a variety of innovative music developed. In terms of when it started, there are several answers.
The most common answer dates it starting post-World War II and lasting up until the present, where it’s still in swing. Postmodernism in music parallels with a bigger shift in postmodernism within the culture, politics, literature, visual art, and dance. Within all these, the main aesthetic is the erosion of boundaries between what is considered serious music, like classical, and what is considered popular or entertainment, like folk, jazz, or break dancing. Postmodernism challenged the status quo. In music, it’s blending this pre-conceived idea that there is a hierarchy in music. I deem some of it a bit in the rebellious sense, against conformity. Postmodernism across the board can be summed up the more or less in the same way contemporary music was summed up. It’s a defining term for all the radical changes that took place in society.
Briefly, postmodernism was and is building against what modernist composers, such as Schoenberg and Stravinsky had done with atonality and the twelve-tone technique in music, by making music even more convoluted with experimental choices for rhythms, harmonies, and abstract sounds and noises. Some composers considered postmodern are John Cage, John Adams, and Iannis Xenakis. One example of what this all looks like is the piece ‘Inuksuit’ by John Adams. The musicians are separated into groups and each group has their own set of instruments. The tempo is about you feeling it on your own and just flowing with your environment. It’s a very percussive piece, and the sheet music itself looks very much like the rock structures it’s named after. For example, one section of the music may have 10 bars across the top, followed by 9 bars under that, 8 bars under that, and so on. Each performer plays at their own pace with a rough guidance from a leader of each group. As you change each instrument you walk to a different location. All of the musicians starting and ending back at the same spot. Getting the picture yet? It’s very experimental. It's much different than hearing a work by Mozart played in a concert hall. Postmodernism in music has a lot of these types of experimenting elements. For example, Iannis Xenakis’s piece ‘Metastaseis’ uses the twelve-tone technique with the duration of the notes based on the Fibonacci sequence.
There were also some more popular bands that were experimenting as well; The Beatles, Pink Floyd, and even Michael Jackson. Michael Jackson was labeled the king of pop for all his innovations in music. The Beatles revolutionized the world with their music; more specifically they were using a lot of innovative techniques in the studio when it came to cutting records. They were tracking stuff playing stuff backwards, speeding up and slowing down the tape, sampling, and using heavy orchestration in some of their songs. This really influenced the future of music as a whole. Keep in mind, this period is less about defining the music and more about just the radical changes that happened all throughout society, but in this case, music.
The postmodern musical period is a continuation of the efforts to expound on the musical creative freedom that musicians and composers felt more compelled to display. It’s almost this ever-evolving need to break all the rules in music. It happens with each musical generation.