Contemporary composers have found several ways to take traditional chords and turn them into new chords. Composers use quartal harmony in which chords are built on intervals of fourths instead of thirds. Moreover, composers use the technique of tone clusters in which several tones are spaced in unusual configurations, including using more dissonant intervals such as major and minor seconds. Composers also use chords with more than five tones in them in contemporary choral music.
Serialism is a technique that composer Arnold Schoenberg developed and refined. It involves taking a set of pitches and repeating them in various ways to prevent any one tone from becoming the center of attention. Unlike classical music, which has a central unifying tone called the "tonic," serialism seeks to break up the pull of a tonal center and create music that is free to move around the chromatic scale.
Polytonality is an attempt at taking several chords from different keys and juxtaposing them upon each other. In polytonality, a composer might use two or more different keys at the same time. This creates dissonant music that uses traditional music theory in a modern and innovative way. A subset of polytonality is bitonality in which a composer only uses two keys. Typically, each instrument will play a related musical idea in a different key. Igor Stravinsky was famous for experimenting with polytonality in his music.
Composers of the 20th century developed a new method of counterpoint. This style of counterpoint creates musical lines that lessen the harsh effects of dissonance and enables tones that would otherwise clash with one other to play together harmoniously.