Half-diminished chords: the m7b5 family

The half-diminished chord is the one that makes a progression sound like a jazz standard or a film score. It goes by several names, m7b5, ø7, "minor seven flat five", and they all describe the same four notes. It looks intimidating on a chord chart and turns out to be one of the friendliest shapes on the neck: four strings, no barre, and a single shape that slides to every key. This page explains what the chord is made of, how it differs from the chords it gets confused with, the shapes worth learning, and the progression it was practically invented for.

What a half-diminished chord is

Start from a minor seventh chord, root, minor third, perfect fifth, minor seventh, and lower the fifth by a half step. That is the whole recipe:

1, b3, b5, b7

Take B. A B minor seventh is B, D, F#, A. Lower the F# to F and you have Bm7b5: B, D, F, A. The D keeps the chord minor, the F makes it unstable, and the A keeps it open at the top.

The name "half-diminished" comes from what the chord contains. Its bottom three notes, B, D, F, form a diminished triad. A fully diminished seventh chord would stack another minor third on top and land on Ab. The half-diminished chord stops at A instead, a whole step above the fifth. Half the way to diminished, in other words. The symbol ø (a circle with a slash) means exactly that, and Bø7 is simply Bm7b5 under another name.

The three chords it gets confused with

The last row is the trap in written charts. Bm7b5 and B7b5 differ by a single letter on paper and by the third in sound: one is a minor chord with a dark fifth, the other is a dominant chord pulling hard toward E. Read the "m" carefully.

The shapes to learn

One shape does most of the work. Put the root on the fifth string, then the fifth on the fourth string one fret higher, the seventh on the third string at the root fret, and the minor third on the second string one fret higher. At the second fret it is Bm7b5: x 2 3 2 3 x. The sixth and first strings stay silent; let the edge of your fretting fingers touch them so they cannot ring.

The second shape puts the root on the sixth string and skips the fifth string: 5 x 5 5 4 x is Am7b5. It is the voicing jazz players reach for because it has the root in the bass and nothing muddy in the middle. Then there are three open-position shapes that are worth knowing because they sit next to the open chords you already play.

Each diagram is read from the thickest string on the left to the thinnest on the right. If any symbol is unfamiliar, how to read chord diagrams covers them all.

The movable shape, root by root

Because the fifth-string shape has no open strings, the fret of the root names the chord. The table is calculated from the open A string, so it is also a method: count half steps up from A and you have the fret.

Where it lives: the minor ii-V-i

The half-diminished chord has one job it does better than anything else: opening a cadence in a minor key. In A minor the second degree is B, and the seventh chord built on it is Bm7b5. Then comes the dominant, E7, and the resolution to Am.

Play the A minor line slowly and listen to the F in Bm7b5 move to the E and G# of E7. That half step is the engine of the whole cadence, and it is why the chord sounds as if it is already going somewhere.

There is a second home, less dramatic. The seventh chord built on the seventh degree of any major key is half-diminished: Bm7b5 in C major, F#m7b5 in G major, C#m7b5 in D major. In a major song it most often opens a short trip to the relative minor: Bm7b5, E7, Am in the key of C is the same cadence as above, borrowed for a moment. The chords in the key of tool lays out every degree for every key.

One last relationship explains a lot of voicings. Am7b5 (A, C, Eb, G) contains the same four notes as Cm6 (C, Eb, G, A). With A in the bass you hear the half-diminished chord; with C in the bass you hear the minor sixth. The minor sixth chords page works through that twin from the other side.

Its relative: the diminished seventh chord

Lower the seventh of a half-diminished chord one more half step and you get a fully diminished seventh chord. C#m7b5 is C#, E, G, B; C#dim7 is C#, E, G, Bb. Every interval inside a dim7 is a minor third, so the chord is perfectly symmetrical: move the shape up three frets and you get the same four notes in a new order.

Where the m7b5 prepares a dominant, the dim7 usually slides between two chords a whole step apart, or stands in for a dominant with a flat ninth. It is a passing chord rather than a destination. The C#dim7 chord page shows it in context.

How to practice it

The fastest way to hear the half-diminished color is to play the minor seventh chord first and then move one note. The fifth drops a half step, and the chord darkens under your fingers.

• Am7 to Am7b5: x 0 2 0 1 0, then x 0 1 0 1 x. The E on the fourth string becomes Eb.
• Dm7 to Dm7b5: x x 0 2 1 1, then x x 0 1 1 1. The A on the third string becomes Ab.
• Bm7 to Bm7b5: x 2 4 2 3 2, then x 2 3 2 3 x. The F# on the fourth string becomes F.

Then play the three ii-V-i lines above, four beats per chord, until the cadence feels like one gesture rather than three shapes. If the change from the half-diminished shape to the dominant is the slow part, the minor seventh chords page and the seventh chords page give you the neighbors to drill.