A Brief History of Western Music
Modern Classical Music
In the 20th century, composers began to experiment more and more with new compositional techniques. Before the 20th century, Western music was almost entirely diatonic: pieces were built using a set of notes known as the major and minor scales. Starting in the 20th century, composers began to make use of other scales, such as the pentatonic and whole-tone scales. Some composers went a step further, and removed the concept of "tonality" altogether: atonal music lacks a tonal centre, and is extremely dissonant as a result.
Examples
Prélude à l'après-midi d'un faune is a symphonic poem by Claude Debussy (1862 - 1918). A symphonic poem is type of program music that is written for orchestra. You can listen to a recording of the piece below. Notice the use of ambiguous tonality throughout the work.
Below is a recording of "Pentatonic Tune", a keyboard piece by Béla Bartók (1881 - 1945). As the name suggests, the piece is built on the pentatonic scale.
Our final example is "Variations for Orchestra", by Arnold Schoenberg (1874 - 1951). Schoenberg uses complete atonality in this piece: the music has no tonal centre, meaning that there is an approximately even distribution of pitches.