In answering this kind of question, theoretical proof is needed. You have to start by asking yourself some useful questions. What do we mean by leading tone? What is the relative major of Ab minor? Music in Ab minor is written or played on which major key?
By now, you must have known that a minor scale (as well as music in minor keys) is played or written on its relative major, which is usually a minor 3rd above. You must have also known that the leading tone is the 7th degree of every diatonic scale. It must a minor 2nd ( a semitone, a half step) below the upper tonic. What you know is always the start point.
Note that we are not talking about parallel minor in this article. That’s a different concept. Parallel keys are any two keys (usually major and minor) that share the same tonic. For instance, F major and F minor share the same tonic (which is F).
Back to the issue! The said key is Ab minor. What is the relative major of Ab minor? Is it B or Cb? If your answer is B, you’re very wrong. Why is it very wrong?
If it were G# minor, it would have been B. Minor third above G# is B. We all know that G# and Ab are enharmonic keys. Enharmonic keys are keys that are practically the same but theoretically different.
For instance, Ab and G# are enharmonic – practically the same (the same key on the keyboard) but theoretically different. You can also say that they are keys that have two names but are of the same frequency. Most piano keys can be spelt enharmonically.
Since the said key is Ab minor, the correct thing to say is Cb not B. Minor 3rd (m3) above Ab can’t be B. Ab – B is an interval of a 2nd (an augmented 2nd to be specific). So, a minor third above Ab is Cb (3 semitones).
Go ahead and build a major diatonic scale on Cb. If you do that properly, using T T St T T T St formula, you’ll have Cb Db Eb Fb Gb Ab Bb Cb‘. That’s to say that the scale of Cb major diatonic is made up Cb Db Eb Fb Gb Ab Bb Cb‘.
|Cb major Diatonic Scale|
Its relative minors will be as shown below. Look carefully, you’ll notice that in the natural minor, the notes are same as those of its relative major. The only difference is that it starts and ends with the 6th degree of the relative major.
The 6th degree of the major becomes the 1st degree of its relative minor. In the harmonic minor, the 6th degree is raised by a semitone. In the melodic minor, the 6th and 7th degrees are raised by a semitone. One thing to note about the melodic minor is that you must restore the two raised notes while playing it descending.
|Ab Natural Minor|
|Ab Harmonic Minor|
|Ab Melodic Minor|
From the above, it can be deduced that Ab natural minor scale is Ab Bb Cb Db Eb Fb Gb Ab‘. The 7th degree (Subtonic tone) is Gb. The 7th degree is technically known as subtonic if it is a major 2nd (a tone, a whole step) below the upper tonic or octave. In this case, it can’t be called a leading tone because it is not a semitone or a half step below the octave.
Diatonic scale is a scale that is made up of series of tones and semitones (five tone and two semitones). That’s why we used that formula T T St T T T St in building the scale of Cb major. A diatonic scale could be major or minor.
There are three forms of minor diatonic: natural, harmonic and melodic. Major diatonic has the structure T T St T T T St, while the natural minor has T St T T St T T. In harmonic minor the 7th tone (7th degree) is raised by a semitone. In Melodic minor, the 6th and 7th tones are raised by a semitone while ascending and restored while descending.
So, if it were to be Ab natural minor scale, Gb would have been the subtonic. Since it is the scale of Ab harmonic minor , G is the leading tone. The same is applicable to Db minor. The relative major of Db minor is Fb not E.
If it were C# minor, it would be E major. Minor 3rd above Db is Fb. If you build a major scale on Fb , you’ll have Fb Ab Gb Bb Cb Db Eb Fb‘.
|Fb major Diatonic Scale|
Its relative minor will be as shown below
|Db Natural Minor|
|Db Harmonic Minor|
|Db Melodic Minor|
For more, read The Concept of Leading Tone and Subtonic, Tonal Harmony: Learning Principles/Prerequisites, The concept of Scale, The Concept of Triad, The Concept of Seventh Chord, The Characteristics and Resolutions of Secondary Dominant Chords, Augmented Sixth Chords, Neapolitan 6th Chord.