Extensions and Alterations

An extension is a chord tone above the seventh in a continuing stack of thirds.

Starting from C7:

The numbers continue past the octave. Degree 2 becomes 9, degree 4 becomes 11 and degree 6 becomes 13.

A plain C9 implies a dominant seventh underneath: C-E-G-B flat-D.

C11 implies the same lower stack through the eleventh. C13 theoretically includes the complete stack through A.

In performance, players omit some inner notes. The symbol still describes the harmonic family and highest named extension.

Cadd9 contains C-E-G-D. The word add includes D without implying a seventh.

C9 contains C-E-G-B flat-D. The plain extension number implies the minor seventh of a dominant chord.

Cmaj9 contains B natural: C-E-G-B-D. Cm9 contains E flat and B flat: C-E flat-G-B flat-D.

No. A full C13 has seven different notes, and playing all seven closely can obscure the chord.

Keep the third and seventh because they identify quality and function. Keep the named extension or alteration.

The root may be supplied by bass. An unaltered fifth is usually optional. The ninth or eleventh may be omitted when 13 is the essential upper color.

In C11, E and F are a semitone apart. A close voicing can make that clash prominent.

Common solutions omit E, omit F, place them far apart or raise F to F sharp for C7 sharp 11.

Omitting E makes the structure resemble a sus chord, such as C9sus4. Name the result according to the notes and harmonic function you intend.

An alteration raises or lowers a chord tone shown in the symbol.

Common dominant alterations are flat 9, sharp 9, sharp 11 or flat 13:

The third E and minor seventh B flat normally remain because they establish C7 as the base chord.

On an equal-tempered keyboard, D sharp and E flat use the same key. Their spelling shows different chord roles.

In C7 sharp 9, E is the major third and D sharp is the altered ninth. Writing E flat would describe a minor third and hide the stacked-extension logic.

Blues-influenced performance may bend pitches between these fixed notes. The chord symbol still uses functional spelling.

Available extensions depend on chord function and scale context.

Cmaj9 can draw from C major. Cmaj7 sharp 11 aligns with C Lydian because F sharp avoids the E-F semitone while adding Lydian color.

A G13 resolving to C can use notes from C major. Altered dominants may draw from altered scale, diminished scale or specific voice-leading choices.

Choose extensions from the progression instead of attaching them randomly.

Altered notes often move by semitone into the next chord.

In G7 flat 9 to C minor, A flat can fall to G while B rises to C and F falls to E flat.

In G7 sharp 5 to C major, D sharp can rise to E. The written spelling makes the intended direction visible.

A strong voicing connects each altered voice to a destination rather than treating tension as a static color.

Put guide tones in a clear middle register and important extensions above them.

Separate dense low notes. Let the bassist carry the root when playing in an ensemble.

For C13, a practical piano voicing might use C in the bass and E-B flat-D-A above. G and F are omitted while the third, seventh, ninth and thirteenth remain.

Build C7, C9, C11 and C13 as complete stacks so you understand the symbols.

Then reduce each chord to root, third, seventh and named extension. Compare the complete and reduced versions.

Finally, play G7 alterations resolving to C major or C minor and follow every altered note by semitone.