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Ok, that changes everything, @Bill_Brown .
So, in the key of C, you have Cmaj7, Fmaj7, Dminor7, and G major. So that would make sense. Only the 1 chord and the 4 chord in any key has a major 7th chord, which has a leading tone. So the 4th note is only one half step away from the root. All the other chords in the key, have a flat 7 chord (people mistakenly call it a dominant 7 chord all the time). The 2 minor, the 3 minor, the 6 minor, and the 7 minor flat 5 all have a minor 7, which means the 4th note is 2 half steps, or frets, away from the root note. Now, here’s the thing, only 1 major chord has a minor 7, and that’s the 5 chord. The 5th degree in a key is called… wait for it… the dominant. So a dominant 7 is any major chord with a flat 7 and it is an out of key chord unless is it the 5 chord of the key you are playing in. The minor 7 chords have a flatted 7 note, but they do NOT have a “dominant 7” note.
So in the above example, if you added the fourth note to the G major chord, it would be an F natural, not an F#, and would therefore be 2 half steps below the root note G, or 10 half steps above the root note assuming G was the lowest note in your chord. A major 7th is 11 half steps above the root and the octave is 12 half steps. A minor 7th is 10 half steps above the root.
So if you wanted to bend the ear, you could play the notes of the Fmaj7 over the C chord, but then play the C7 over the F, which would mean throwing some Bb’s in there just for fun. You ear would hear F major for that moment. But then make sure you emphasize the B note in the G chord and the Cmaj7 chord to pull the ear back into C major.
And then there’s always the “blue” note. Notice the the blue note is “between the 4th and 5th”. But slow down, that’s not helpful at all unless you mention it’s the augmented 4th in the minor scale. It’s actually between the 2nd and 3rd in the major scale. Since I relate everything back to the major scale, that is how I remember it. So in the key of C, if you play the Eb it sounds very cool. But then, you can play literally any note if you put into the right context. The flatted notes will give is a bluesy feel. Why not sharped notes? Simple, you have sharps in most major scales. But F and C become sharped right away in the progression, G major and D major. So, C minor and F minor happen in the flat keys.
All a matter of perspective. But that’s why those notes bend the ear without making is sound “wrong”, because they come from the flat keys.
MG 😀