C#majb5

Construction

Built from these intervals: 1-3-b5.

The 3 sits four half steps above the root, keeping the chord's bright, major quality. The b5 drops the perfect 5th a half step to a tritone (the interval of six half steps from the root), trading grounding for instability -- the one change that makes a major triad a majb5. That b5 is enharmonically the #11 (an octave and a tritone above the root), giving the chord its Lydian color -- Lydian being the major scale with a raised 4th.

Harmonic Function

In Roman numeral analysis (uppercase = major, lowercase = minor):

• Imajb5 -- tonic major triad tinted with the b5 (enharmonic #11): a slice of Lydian color, used for its sound more than for a cadence
• IVmajb5 -- on the fourth degree the b5 is the key's leading tone (a half step below the tonic), the chord's natural Lydian home, as in a IVmajb5-I resolution
• Passing chord -- appears when a major triad's perfect 5th slides down a half step through the b5

Character

Bright but ungrounded. The major 3 keeps a plain major chord's optimism, but the b5 tritone pulls the floor out. Compare to maj (1-3-5): identical but for the lowered 5, trading the perfect 5th's grounding for tritone unrest.

Related Sounds

These are majb5's closest relatives -- each sits a single interval away:

• maj (1-3-5) -- the parent triad; the natural 5 grounds it where the b5 destabilizes
• maj7b5 (1-3-b5-7) -- adds the major 7th on top; majb5 is maj7b5 without it, sharing the same #11 Lydian color
• aug (1-3-#5) -- the fifth raised, not lowered; both alter a major triad's 5 in opposite directions, and both are whole-tone triads (root, 3, and altered 5 all sit in the whole-tone scale)
• dim (1-b3-b5) -- the same b5 tritone, but a b3 in place of the major third; that major third is all that keeps this from being diminished

Voice Leading

Voice leading tracks how notes move between chords. The b5 is a tritone; splitting the octave evenly, it pulls equally either way. These movements hold from any root.

• IVmajb5 to I: the leading-tone b5 rises a half step to the root of I; the root falls a half step to the 3; the 3 falls a whole step to the 5.
• Imajb5 to I: the b5 rises a half step back to the natural 5, restoring the stable major triad; the root and 3 stay as common tones.
• Imajb5 to V: the b5 rises a half step to the root of V; the root falls a half step to the 3; the 3 falls a whole step to the 5.

Practice Seeds

Build the tritone. Play the root, the 3, then the b5 in turn -- the major 3 stays bright while the root-to-b5 tritone removes the floor. Trains your ear to hold major color without grounding.

maj vs. majb5. Play a major triad, then lower only the 5 a half step. Trains your ear on that single drop -- the perfect 5th's stability giving way to tritone tension.

Lydian leading tone. Play IVmajb5 resolving to I, following the b5 as it rises to the tonic. Builds hearing for the chord's main role: Lydian color on the subdominant pulling home.

Major or diminished. Play majb5, then lower the 3 to b3 to land on a diminished triad. Sharpens the major-third-versus-b3 distinction -- same b5 tritone, opposite quality.