Seventh scale degree, or leading-tone, leading to the first scale degree, or tonic, in C major .
Tonic and leading tone chords in C  . C major and B diminished (b°) chords.
Dominant seventh and incomplete dominant seventh in C major: G7 and b° chords  .
Tritone resolution inward and outwards . Both notes resolve by half step.
Tritone substitution, ii-subV-I on C, creates an upper leading-note (D, which leads down to C)

In music theory, a leading-note (also subsemitone, and called the leading-tone in 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.

Cadence featuring an upper leading tone from a well known 16th-century lamentation, the debate over which was documented in Rome c.1540 (Berger 1987, 148).

Seventh chord resolution from Scott Joplin's "Maple Leaf Rag" (Benward & Saker 2003, 203)  . Note that the seventh resolves down by half step.

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 note 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 works from the 14th and 15th century Western tradition, the leading-note is created by the progression from imperfect to perfect dissonances, 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 note.

As a diatonic function the leading-note 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 called instead, the subtonic.

Leading-tone seventh chord in C major: viiø7  .
Leading-tone seventh chord in C minor: viio7  .

The leading-tone seventh chords are viiø7 and viio7,(Benward & Saker 2003, 219) in major and in minor.