Many great chord progressions in tonal music are based on specific sequences of triads and chords. These progressions sound naturally fulfilling or appealing. These chords often all consist of notes that strictly come from the major or minor scales they are based on. They are considered diatonic chords. When musicians start speaking about chords using numbers instead of letters, this is what they are talking about. Listen and learn more about the diatonic triads and 7th chords from the major and minor scales.