In music theory, a leading-tone (called the leading-note outside the US) is a note or pitch which resolves or "leads" to a note one semitone higher or lower, being a lower and upper leading-tone, respectively.
Generally, the leading tone is the seventh tonal degree of the diatonic scale leading up to the tonic. For example, in the C major scale (white keys on a piano, starting on C), the leading tone is the note B; and the leading tone chord uses the notes B, D, and F: a diminished triad. In music theory, the leading tone triad is symbolized by the Roman numeral vii°.
According to Ernst Kurth (1913) the major and minor thirds contain "latent" tendencies towards the perfect fourth and whole-tone, respectively, and thus establish tonality. However, Carl Dahlhaus (1990) shows that this drive is in fact created through or with harmonic function, a root progression in another voice by a whole-tone or fifth, or melodically (monophonically) by the context of the scale. For example, the leading tone of alternating C chord and F minor chords is either the note E leading to F, if F is tonic, or A♭ leading to G, if C is tonic. In the 14th and 15th centuries, the leading-tone is created by the progression from imperfect to perfect consonances, such as a major third to a perfect fifth or minor third to a unison. The same pitch outside of the imperfect consonance is not a leading tone.
As a diatonic function the leading-tone is the seventh scale degree of any diatonic scale when the distance between it and the tonic is a single semitone. In diatonic scales where there is a whole tone between the seventh scale degree and the tonic, such as the Mixolydian mode, the seventh degree is the subtonic.