A half-diminished chord

What each note does

• A is the root. It names the chord and is the note your ear hears as home.
• C is the minor third, a half step below the major third (C♯). That one note is the whole difference between sad and happy.
• E♭ is the flat fifth. It pulls the chord tight and unstable, which is why diminished chords want to move somewhere.
• G is the flat seventh. It adds tension that pulls toward the chord a fourth above, the sound of blues and of a V chord heading home.