B diminished 7th chord

What each note does

• B is the root. It names the chord and is the note your ear hears as home.
• D is the minor third, a half step below the major third (D♯). That one note is the whole difference between sad and happy.
• F is the flat fifth. It pulls the chord tight and unstable, which is why diminished chords want to move somewhere.
• A♭ is the double-flat seventh, the same pitch as a sixth. Stacked on the flat fifth it makes the chord fully diminished.