A9 Chord

Build A-C sharp-E-G for A7, then add B a ninth above the root. The complete spelling is A-C sharp-E-G-B, formula 1-3-5-flat 7-9. The ninth shares its pitch class with the second, but is normally voiced higher to leave space above the seventh chord.

Here, C sharp is the major third, G is the minor seventh, and B is the ninth. Those three notes tell you more about the chord's identity than the fifth alone. On a keyboard, try leaving out E after you have learned the full five-note spelling: the symbol remains A9 in a common abbreviated voicing.

With five different chord tones, there are theoretically four inversions. The piano control offers root position and the first three. Their note orders are:

Move the lowest note up an octave each time. A fourth inversion would put B in the bass, though the current control does not offer that button. In a real arrangement, the bass instrument may choose another note independently of the keyboard hand.

Aadd9 has the root, major third, fifth and ninth, but no seventh. A9 adds G, the minor seventh. Play Aadd9, then add that one note. The difference is structural, not just a richer version of the same symbol: the plain 9 implies the seventh. Read Ninth Chord for the broader notation rule.

In D major, A9 can serve as an expanded dominant on the fifth scale degree before a D chord. Play A7-A9-D major. The ninth changes the color while the third and seventh keep the dominant framework. In blues or other styles, the same chord shape may appear without an immediate textbook resolution, so listen to the surrounding harmony before naming its job.

Play A7 and A9 with the same root and volume. Sing B when it appears, then remove it without changing the other notes. Finally compare A9 with Aadd9: this time listen for G, the seventh. These two one-note comparisons make the chord symbol easier to remember.