Tension

In jazz harmony, a color tone added to a basic chord as a 9th, 11th, or 13th, including altered forms. Over Cmaj7, D is 9 and A is 13; in a C Lydian context, F-sharp is sharp 11. Available tensions retain the intended chord sound and function. Their suitability depends on the key, chord scale, melody, and voicing, not just the chord symbol.