In music or music theory, the supertonic is the second degree of the scale. For example, in the C major scale (white keys on a piano), the mediant is the note D; and the supertonic chord uses the notes D, F, and A. In music theory, the supertonic chord is symbolized with the Roman numeral II if major or ii if minor.
"Supertonic" also refers to a relationship of musical keys. For example, relative to the key of C major, the key of D major is the mediant. Modulation (changes of key) to the mediant are relatively rare, compared with, say, modulation into the dominant.