Magnetic Semitones

Perhaps you will have already noticed a symmetry within the major scale.
The first four notes being identically constructed as the second four.
These are often referred to as the two tetrachords.
Each consists of two tones and one semitone, and are separated by the interval of a tone.

As in living beings the seed, as it were, for the next is contained within the first. We see travelling clockwise, that the second tetra chord of any scale is the building block for the next scale around the Mandala, and requires the sharpening of the seventh note in order for it to become stabilised as a new key in its own right. Why is this?

Without the sharpening of the seventh degree, the second half of this new scale would now have the semitone in a odd position, and would still sound to us like part of the "parent key." We would "Psycho-acoustically" not recognise any different key basis.
The new scale would simply sound like the “parent key” starting on the dominant.

This is because it is only the positioning of the semi-tones within a scale that give us the reference points with which we determine the key, or imply the tonic, even if it is not actually sounded. We learn, and remember, sub-consciously, that all scales, major and minor, end with a semitone leading up, and back home to the tonic.
This is a powerful reference point for our ears, and in conjunction with the other semitones that are presented to us during the course of a melody, we deduce a sense of key.

This accounts for the floating quality of whole tone music.
A chromatic scale or whole tone scale have no innate key structure (each step being identical) merely a starting note.

The magnetic quality I refer to can be demonstrated if we look at the two semitone movements in a dominant seventh to tonic cadence, or V7 - I.
The leading note will be attracted back to the tonic, and the sub-dominant will be attracted back to the mediant.

This phenomenon also accounts for the ubiquitous usage of a diminished seventh chord, sometimes referred to as the "Charring Cross" chord, for you can modulate to virtually
anywhere from it and modulate to it from virtually anywhere, because of the availability of possible semitone movements to other recognisable chords. In other words - you cannot move any of the individual notes contained in a diminished seventh chord by a semitone, in either direction, without creating another recognisable chord.

Developing a sensitivity towards hearing the magnetism of semitones will help immensely your sense of understanding and feeling for harmonic progressions as they lead you through a piece
of music.