Leading tone
The leading tone is the note one semitone below the tonic, the seventh degree of the major scale, and its pull up into the tonic is what makes the dominant chord want to resolve.
Where is the leading tone?
One semitone below the tonic, on the seventh degree:
- C major: B, resolving up to C.
- G major: F♯, resolving up to G.
- A minor: G♯, resolving up to A.
It sits inside the dominant chord as its third. In C major the dominant is G–B–D. Add the seventh and you get G7, G–B–D–F, where two notes resolve at once: B rises to C, and F falls to E. That double semitone move is most of what you hear at the end of a piece.
How does it sound?
Sing up a major scale, Do Re Mi Fa Sol La Ti, and stop on Ti. The urge to sing one more note is the leading tone. A semitone is the smallest step there is, so the ear hears that note as almost home.
Why does the minor key need a G♯?
The natural minor scale has no leading tone. In A minor its seventh is G, a whole step below A, and a whole step does not pull the same way. So minor music usually raises the seventh to G♯, which turns the dominant from E minor into E major. That raised note is the reason the harmonic minor scale exists.
What is it confused with?
The subtonic: the seventh degree when it lies a whole step below the tonic, like G in A minor or B♭ under C. Same position in the scale, but without the semitone it has no pull. Only the note a semitone below the tonic is a leading tone.
Try it: The pull of the dominant plays progressions with the last chord missing so you can hear where the leading tone points, and the minor key shows the G♯ at work.