Diatonic Harmony

Hi,

The term diatonic means that the notes come from the same key.

Chords and notes that fall outside those made from these scales are not diatonic.

In the key of G major,the seven notes are– G, A, B, C, D, E, and F#.

A Cmaj7 is a diatonic chord in the key of G major since the notes are - C E G B which fall inside the G major scale.

A Cm7 is not a diatonic chord in the key of G major since the notes are - C Eb G Bb,
where Eb and Bb fall outside of the G major scale.

Hope this is clear.

Thanks