A 9th chord
A9 chord (also called Dominant 9th chord) for piano presented by keyboard diagrams.
Explanation: The A ninth is a five-note chord. Due to practical circumstances, however, the fifth (E) is often omitted and/or the chord is played inverted. The chord is often abbreviated as A9.
Theory: The A ninth chord is constructed with a root, a major third, a perfect fifth, a minor seventh and a major ninth.
Notes: A - C# - E - G - B