Tensions and pedal · Music theory
Ninths, elevenths and thirteenths: which can be added to each chord, altered dominants, the notes that cannot be left out, and the pedal point.
Beyond the seventh
If we keep stacking thirds on a seventh chord, the 9th, 11th and 13th appear. They are the same notes as the 2nd, 4th and 6th, but an octave higher, and in the chord they act as an addition: they do not change its function, they give it color. They are called tensions.
The dominant ninth was already in use in the Classical period, and elevenths and thirteenths spread in the 19th century and with Debussy. In jazz they are the norm, and a seventh chord without tensions sounds almost austere.