Why does Jazz sound so weird and funky?? In a nutshell, it's harmony on steroids.
Jazz spawned as the offspring of impressionistic harmony and is a natural evolution of the entire western musical harmonic language. In layman's terms, they just kept adding more notes to chords because they got bored.
In the late 19th Century, composers pushed musical structure to its eventual limits. Jazz, while technically speaking was birthed in the US (and has many rhythmic and cultural elements too), is really just an extension of western European music.
Again, put simply, chords finally became boring after centuries of use and so evolved by adding more notes. In musical terms, chords are built in thirds. A basic chord in music consists of three notes, known as a triad. New composers felt the need to add more notes on top of these three in order to create new sounds and harmonies that excited the listener.
Here is a good primer for definitions of chord qualities and how they sound. This musician has a great YouTube channel for people who want to learn piano.
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Original blog post @ thesecretsofmusic.com