I'm failing to understand why this is so difficult for me to understand when I play two other instruments where I have little trouble with chordal theory. The question I have is this: in my jazz book there are two examples of the diatonic major scale. One is in the key of C, the other G. Now these both have within them the chord of D, in the case of the C scale, it's ii, the G scale, it's on the V. My problem comes because from my understanding of chord theory, the D chord has the flatted third with the F, making it minor. So how in the key of G can this chord suddenly be major without there being an F# in it? And why does the F#diminished chord in the key of G get written with a F as the starting point in standard notation when it's based on the vii dim which in this case is a sharped note. The more I read, the more I get confused.