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Thirteenth (13th) – Definition
A thirteenth (13th) is a compound musical interval equal to an octave plus a sixth. In modern harmony, it is most commonly encountered as the highest extension of a seventh chord, adding richness and color to the harmony.
What is a Thirteenth?
A thirteenth spans thirteen letter names. Like other compound intervals, it can be viewed as the corresponding simple interval—in this case, a sixth—raised by one octave. In chord symbols, a thirteenth is written as 13, such as C13.
Thirteenths are especially common in jazz, blues, soul, and contemporary harmony, where they add a smooth, sophisticated sound without changing the basic harmonic function of the chord.
Why is the Thirteenth important for pianists?
Learning to use thirteenths helps pianists create richer chord voicings and more colorful accompaniments. Because the interval is so wide, the 13th is usually included as part of a chord voicing rather than played as an isolated interval.
Understanding thirteenths enables pianists to interpret advanced chord symbols confidently and perform more authentic jazz, pop, gospel, and soul piano.