Here are a few observations about the diatonic scale:
- The diatonic scale could be viewed as selecting seven notes from the chromatic scale. This leaves 5 notes that aren't used. These five notes that aren't used are the pentatonic scale used in Chinese and rock'n'roll music.
- Another interesting relationship of the pentatonic scale to the diatonic scale: In a minor key (let's use Aćminor since it has no accidents), not only are there minor triads at I, IV, and V (A, D, and E), but you can play a pentatonic scale with these notes as the root. So the diatonic scale can be deconstructed into three overlapping pentatonic scales.
- Don't know where to modulate? Try modulating completely out of the scale into the diminished scale by going to the VII and playing the diminished scale (in C major this would be changing the scale to [B, D, F, Ab], or playing four select notes of the Locrian mode, but with the fourth note (A) lowered to (Ab)) Then when you have played it for a while you can exit the diminished scale by playing a major scale one half step up from wherever you are. Alternately, you could then go from the diminished scale into a major-minor scale rather smoothly.
- The diabolus in musica (tritone, diminished fifth, augmented fourth) could be viewed as what's missing from a pentatonic scale that would make it a diatonic scale. I think if you were to view the pentatonic scale as feminine (this is only one viewpoint, do not take this so seriously), it is obvious what the tritone represents to make the diatonic scale masculine >
- Tired of the Aeolian mode (Minor scale)? Try playing the Phyrgian mode, which is exactly the same notes but with a second note that is lowered by a half step (C, Db, Eb, F, G, Ab, Bb) or (C, C#, D#, F, G, G#, A# for trackers). It is, in my opinion, the strongest sounding mode of the diatonic scale, evoking force in the restlessness caused by the first note of the scale being a leading tone to the second note.
[Comments, suggestions, corrections, or additional observations welcome!]