An extended chord is a chord obtained by adding to a triad a note which is more than an octave above the root. Extended chords include the ninth, eleventh, and thirteenth (see compound interval). Each extended chord belongs in one of three types – dominant, major, and minor. The tonality of the third and seventh intervals determines the totality of the extended chord.
The thirteenth is the farthest extension diatonically possible as, by that point, all seven tonal degrees are represented within the chord. In practice however, extended chords don't use all the chord members. For instance, when it isn't altered, the fifth is often omitted, as are notes between the seventh and the highest note (i.e., the ninth is often omitted in an eleventh chord; the ninth and eleventh are usually omitted in a thirteenth chord), unless they are altered to give a special texture.