What are the differences between modern and postmodern music?

Modernism and Postmodernism are distinct periods within the history of western European music. In order to understand their differences, it is important to know how they fit within the context of music history. The major periods in western European music history are: Medieval, Renaissance, Baroque, Classical, Romantic, Modern, and Postmodern, in chronological order. Since modernism follows the Romantic era, it can be seen as a reaction to romanticism; postmodernism is, in part, a reaction to...

Modernism and Postmodernism are distinct periods within the history of western European music. In order to understand their differences, it is important to know how they fit within the context of music history. The major periods in western European music history are: Medieval, Renaissance, Baroque, Classical, Romantic, Modern, and Postmodern, in chronological order. Since modernism follows the Romantic era, it can be seen as a reaction to romanticism; postmodernism is, in part, a reaction to modernism in music. 

The tonal system we use in music was refined into its modern form in the Baroque era, under the influence of J.S. Bach. This system remained in ascendance until the end of the Romantic era when Wagner and others began to expand tonality. The Romantic era also featured music that depicted emotions. 

As a reaction to romanticism, modernist composers began to expand tonality radically (Stravinsky) and question the need for privileging one note over another the way the first note of a scale is privileged in tonal music (12-tone composers, such as Schoenberg). Some modernists reacted to the emotionalism of romanticism by creating austere music (Webern) and others took that emotionalism to the extreme (Stravinsky, particularly in Rite of Spring). 

Postmodernist composers reacted to modernism by creating music that contrasted with that of the high modernist composers. The postmodern ethos allowed artists of all kinds (architects, visual artists, composers) to draw from all historical periods to create a pastiche, instead of simply rejecting music of prior eras. As a result, composers such as John Adams create music that has a stronger tonal feel to it. 

These are generalizations about tendencies within each era; it is important to look at the details and listen to a lot of music from each era to get a feel for similarities and differences.