Changes

Note that simply because the root of a chord is in the scale doesn't mean that any chord with that root is diatonic to the key. Gm, A, and Bm all have roots in C Major but are ''not'' diatonic to C Major. Gm has a BbB{{flat}}, A has a C#{{sharp}}, and Bm has a F#{{sharp}}. BbB{{flat}}, C#{{sharp}}, and F# {{sharp}} are not found in C Major.

Let's compare C Major to G Major, which has the exact same notes as C Major except an F# {{sharp}} instead of F. Changing that one note (F -> F#{{sharp}}) affects 3 of the chords. B° B{{dim}} becomes Bm, Dm becomes D, and F becomes F°F{{dim}}. Notice that G, Am, C, and Em are diatonic to both C Major and G Major.