A7b5

Construction

Built from these intervals: 1-3-b5-b7.

The 3 supplies dominant brightness while the b7 adds the restless pull toward resolution. The b5 replaces the natural 5, creating two tritones (the interval of six half steps): one between 1 and b5, another between 3 and b7. This double-tritone structure makes the chord exceptionally unstable. Dom7 with the 5th lowered a half step -- an alteration (a chord tone raised or lowered by a half step from its natural position) that doubles the chord's internal tension.

Harmonic Function

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

• V7b5 -- altered dominant resolving to I. The b5 intensifies chromatic tension beyond a standard V7, giving the resolution a sharper edge.
• bII7b5 -- tritone substitution for V7. Tritone substitution means replacing a dominant chord with another whose root is a tritone away; they share the same 3-to-b7 tritone, so both resolve to the same tonic. Here the bass descends by half step to I instead of dropping a fifth.
• Tritone pivot -- because 7b5 contains two tritones, it can resolve in multiple directions. A 7b5 built a tritone apart from another shares all four pitch classes, enabling one to substitute for the other.

Character

Unstable and sliding. As a member of the altered dominant family, the double tritone gives this chord a slippery, chromatic quality -- it wants to move by half step in every voice. Compare to 7#5: both alter the 5th of dom7, but 7b5 compresses inward (diminished 5th) while 7#5 stretches outward (augmented 5th). This is the sound of jazz tritone substitution -- harmony that creates surprise through chromatic approach rather than diatonic function.

Related Sounds

These chords share the dominant core -- each changes one element:

• 7 (1-3-5-b7) -- natural 5th, more stable and grounded; the parent chord before the alteration.
• 7#5 (1-3-#5-b7) -- raised 5th instead of lowered, augmented brightness rather than diminished tension.
• dim7 (1-b3-b5-bb7) -- shares the b5 but with b3 and bb7 (enharmonically the same pitch as the natural 6), creating fully symmetric diminished function.
• 7(b5,b9) (1-3-b5-b7-b9) -- adds the b9 on top, maximum altered tension toward minor.
• Pairs with the whole tone scale or the Lydian dominant scale (fourth mode of melodic minor), depending on context. The altered scale (seventh mode of melodic minor) also contains the b5.

Voice Leading

Voice leading tracks how individual notes move from one chord to the next. Half-step motion defines this chord's resolutions -- nearly every voice has a chromatic path to a chord tone in the target.

• V7b5 to I: The 3 moves up a half step to the root of I. The b5 moves down a half step to the root of I. The b7 moves down a half step to the 3 of I. The root of V holds as the 5 of I.
• bII7b5 to I: The root descends a half step to the root of I. The 3 descends a half step to the 3 of I. The b5 is already the 5 of I (common tone). The b7 ascends a half step to the root of I. Tritone substitution in action -- the same target chord reached from a different bass note.
• ii7 to bII7b5 to I: The bass descends chromatically -- each root a half step lower. The b3 of ii holds as the 3 of bII (common tone), then resolves down a half step to the 3 of I.

These movements apply in any key -- the intervals are the same regardless of root.

Practice Seeds

Find both tritones. Play 7b5 and identify the two tritone intervals (1 to b5, and 3 to b7). Understand why this chord is more unstable than dom7 -- two competing pulls instead of one.

Tritone substitution swap. In any key, play V7 resolving to I, then play bII7b5 resolving to I. Hear how both progressions resolve to the same target through different bass motion -- this is tritone substitution in action.

Chromatic bass descent. Play ii7 to bII7b5 to I in several keys and listen to the bass line stepping down by half steps. Train your ear to hear the seamless chromatic descent that makes this substitution so effective.

Compare fifth alterations. Play 7, then 7b5, then 7#5 on the same root. Distinguish the three fifth qualities -- natural, diminished, augmented -- and how each changes the dominant's character.