The Mu Chord

Major triad + 9, no 7 — the colour-without-function chord, twelve keys

The mu chord is a major triad with the 9 added but no 7. C mu = C E G D. The crucial detail: no 7th. Adding a maj7 would make it a maj9 (with a tonal-I quality); adding a ♭7 would make it a dom9 (a V-going-to-I quality). The mu chord keeps the 9 colour without committing to either function. It just rings.

The voicing here: LH plays the root, RH plays a triad of 9-3-5 (close-position major triad starting on the 9). For Cmu: LH C, RH D-E-G. The 9 (D) at the bottom of the RH cluster is the chord's defining sonority — neither a chord tone (it's an extension) nor a leading tone (it doesn't pull anywhere).

LH

Root.

RH

9 + 3 + 5 (close-position triad starting on the 9).

Cmu add9, no 7

Fmu add9, no 7

B♭mu add9, no 7

E♭mu add9, no 7

A♭mu add9, no 7

D♭mu add9, no 7

F♯mu add9, no 7

Bmu add9, no 7

Emu add9, no 7

Amu add9, no 7

Dmu add9, no 7

Gmu add9, no 7

The Daily Drill

Add the 9, omit the 7

3 minPlay Cmu and let it ring. The chord is just C major with a D on top — but the D added at the bottom of the RH (the 9), instead of the top, creates the defining sonority. Without the 7 the chord feels neither maj7-major nor dom7-functional.
8 minCycle three keys. The RH triad stays close position throughout (the chord's three top notes are always 9-3-5, all within a 4th of each other).
4 minAdd a maj7 to the chord and listen to it become a Cmaj9 — much warmer, more committed to being a tonic. Remove the 7 again and the mu chord returns — brighter, less committed, more "hovering."
3 minApply: play the chords of Aja's opening (mu chords moving stepwise) or Peg's verse (mu chords with bass motion). The Steely Dan sound depends on this specific voicing.

Why no 7?

The 7th of any chord — whether major 7 or dominant ♭7 — pulls the chord into a tonal function. Major 7 says "I am the tonic"; ♭7 says "I am the dominant pulling toward I." Omitting the 7 entirely (while keeping the colourful 9) leaves the chord open — neither resolved nor pulling toward resolution. The mu chord exploits this gap. It's the chord pop and fusion needed when they wanted jazz extensions without jazz function. As for the name: Becker and Fagen never gave a serious explanation — by the accounts that exist it started as a joke, and the exact reason is forgotten.

Major triad + 9 + nothing else. Three notes plus a root. The chord that defined the Steely Dan sound and seeped into half of 1980s smooth jazz.

80 bpm

Sound