Chord extensions refer to adding to, or extending, a triad. This includes 7ths, 9ths, 11ths, 13ths and their alterations. Knowing how to voice extended chords is crucial to jazz ensemble writing. Here are a few general rules to follow:
Abene's voicings below are very dense. The first, third, and seventh bars use six-part density; the fifth bar is seven-part density. Notice the use of the M7th intervals.
Sometimes chords can not be easily understood using traditional chord symbol notation. In this case polychord symbols should be used. Polychords are written using a horizontal line to separate the two chords, indicating both chords are to be played together. This is different from inverted chords which use a diagonal line to indicate a bass note other than the root.