A7 chord substitution confusion

HI. I just joined the forum. I don't know anyone's level of theory knowledge so please allow me to put in my 2 cents rearding A7 (dominant 7) chord substitution. I would recommend that study should be done regarding the tritone, once I understood this it helped me a great deal with chord substitution.

Chord subs can get crazy if you try and understand or even read about it all at once. Dominant 7 chords contain a tritone.
Simply put, this is the 3rd and 7th of the Dominant 7 chord. A7 would contain the notes A C# E G

A is the tonic or root, C# is the Major third, E is the Fifth (perfect fifth) and G is the b7 (its technical name is the minor 7th but don't let that confuse you, its not a minor chord because the third is Major. Anyway, for all intents and purposes, lets call it the b7 (flat 7)

This explanation really requires an understanding of 2 note intervals and I don't know anyone's knowledge in this area. I will elaborate a little. Before I get back to A7 and chord substitutions,the best way to understand intervals is relating all notes of the major scale to the root note. I used A because that is the subject of this thread.

A Major scale: A B C# D E F# G#.

A to B is a Major second- A to C# is a Major third- A to D is a Perfect fourth- A to E is a Perfect fifth- A to F# is a Major sixth- A to G# is a Major 7th. (notice A dominant 7 has a G, not a G#) Anytime you flat a Major interval it becomes a minor interval, hence, this is why the A dominant 7 chord contains a minor 7th interval.

If you invert an interval, you simply change the notes places.
A to B inverted is B to A
A to C# inverted is C# to A
A to D inverted is D to A
A to E inverted is E to A
A to F# inverted is F# to A
A to G# inverted is G# to A

Ok, here is the part that I will bet nobody ever told you. To calculate the name of an interval if you invert it, the process requires simple math, nothing more. Let me tell you how this is done. Think of it this way. All intervals have to add up to the number NINE when you invert. Also inverting reverses the name (Major becomes minor, and minor becomes Major. Perfect remains perfect but all 3 types always add up to 9 when you invert, here is my explanation for this.

As I said A to B is a Major second, if you invert it, it becomes a minor interval. Think 9... As it stands now, it is a MAJOR second, or Major 2 Think of what you would add to the number 2 to make it 9. You would add 7.

So Major 2 intervals become minor 7 intervals when inverted. (Major becomes minor, minor becomes major)

The Major 3 interval, add 6 to 3 and you get 9. Major 3 intervals become minor 6 intervals when inverted.

Perfect 4th intervals become Perfect 5th (perfect always is perfect) 4 and 5 = 9

Perfect 5th intervals become perfect 4th when inverted, 5 + 4 = 9

Major 6 becomes minor 3 (6 + 3 = 9)

Major 7 becomes minor 2 (7 + 2) = 9

The reason I had to explain all of this is because the tri-tone is the ONLY interval (other than the octave) that does NOT change its value, it becomes a tritone even when inverted.

Back to A7. A7 is A C# E G The tri-tone is ALWAYS the 3 and b7 of a dominant 7 chord. C# to G is a diminished 5th. (flatting a perfect interval makes it a diminished interval and sharping a perfect interval makes it an augmented interval.

The diminished 5 is also called a flat 5. You have probably heard of b5 substitution, well this is really cool because if you invert the C# and E in the A7 chord, you get E and C# (this is an augmented interval, but the cool part is that it is STILL A TRITONE.

This is how flat 5 subs work. Take the C# and E and change the root to a flat 5 of A. (Eb would be the note.

Point is that A C# E G is an A7 chord. If you change the A to an Eb it becomes an Eb7 chord. The values of the tritone simply reverse their value. C# is the 3 of A G is the b7 of A C# (Db) is the b7 of Eb and G is the third.

If any of this sounds like another language, I would recommend studying intervals and chord construction. Basically there are 4 chord types. Major, minor, Augmented and diminished.

Jazz players consider Augmented and Diminished subs for the dominant 7 chord so many jazz musicians simply see three chord types, Major minor and dominant. Confused yet, well don't be bummed out if you are, trust me, once you understand what a tritone is and how it works with dominant 7 chords, you will see that much of this will make more sense.