A dominant 9th 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 major third. It makes the chord sound bright. Lower it a half step to C and the chord turns minor.
• E is the fifth. It adds weight without changing the character, so it is the first note to drop when a voicing gets crowded.
• 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.
• B is the ninth, the second an octave up. It opens the chord and adds color without changing its function.