Major and Parallel Minor Scales.
A) A “parallel” scale means it has the same starting and ending notes as another scale. The major scale is generally used as the starting scale, which can be modified to create many parallel scales. The names get esoteric; historically a variety of names have been attached to the same scales, and it can get overwhelming on a theoretical level. An efficient approach is to get familiar with the actual intervals in the scales, no matter what you name them. Then you will find the easiest way for your particular brain to organize them. For example, many people think of the melodic minor scale as “just the major scale with a lowered 3rd.” However, some people (possibly ones who have conspicuously neat apartments and arrange their soup cans alphabetically), prefer to think of the melodic minor as the natural minor scale with a raised 6th and raised 7th, and that it should only be played ascending. I try not to judge; no method is better than another. Ideally, we should be able to understand music in all its theoretical descriptions, but practically speaking each of us finds our own personal path to understanding.
B) It can be helpful to organize scales in parallel, with the same root note. I find it logical to arrange them as follows: Major, Melodic Minor (b3rd), Dorian (b3rd, b7th), Harmonic Minor (b3rd, b6th), Natural Minor (b3rd, b6th, b7th). Here I’m using the Major scale as the starting point and altering tones to create the parallel scales listed above. There are of course many more parallel scales to be constructed, but this is a good starting point, a total of 5 scales.
C) I recommend locking in the root, 4th and 5th of the major scale. Get those intervals firmly in your ear, and then add the 3rd and 6th and 7th. Finally, add the 2nd. When you create the other 4 parallel scales, the 4th and 5th are static, stable tones that help define all the above scales. The 3rd tells your ear if the scale is primarily major or minor. Try shifting between the major 3rd and minor 3rd, paying attention to the way you hear the other intervals, as you add in the 6th, 7th and 2nd. If you just play a complete scale, it may be good for your finger dexterity, but it may be too much information for your ear to process. Slow down and play smaller pieces of each scale to hear how they differ from each other.
D) Play short phrases in the major scale, then alter them to the parallel scales. This will help your ear recognize the scales in practice, and will also help your improvisational versatility. For example a phrase using the major 3rd sounds quite different when the 3rd is lowered, even though the body of the phrase and the fingering techniques are almost the same. The devil is in the details, as they say.
E) Work your way through the Circle of 5ths/4ths in this way. It doesn’t have to be an exhausting chore, but rather a musical meditation. The ergonomics of each scale present challenges but also finger patterns that feel interesting and will stimulate your creativity, especially for improvisation. Also, the tonal qualities of your instrument, the actual vibrations, will feel different in each key, and the intervals of the various parallel scales can have different emotive power in different keys. For example, on tenor sax when I play the G dorian minor (F concert), the 6th is an E (D concert). This note speaks well in the upper register, it really sings, and often changes the way I approach other notes in the scale.
F) What next? There’s always more, but remember to go slow, so that your fingers, ears and brain can work together. After you’ve gotten familiar with the above 5 parallel scales, trying playing the diatonic triads, then the diatonic 7th arpeggios. And then of course there are the 7 modes of each scale. But first make yourself a nice cup of tea.