How to Use Music Theory to Build Diminished Triads

By Michael Pilhofer, Holly Day

Music theory gives musicians the necessary elements for building diminished triads. Diminished triads are minor triads that have had the fifth lowered a half step. Diminished triads are stacks of minor thirds with three half steps between each interval.

You can build a C diminished triad (written as Cdim) by counting out the half steps between intervals, like this:

Root position + 3 half steps + 3 half steps (6 half steps above root)

Check out C diminished on the keyboard.

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Here is C diminished on the staff.

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Using the method of starting with the major key signature first and then building the chord, the formula you want to remember for building diminished chords is

Diminished triad = 1 + flat 3 + flat 5

So the first major scale degree stays the same, but the third major scale degree and the fifth major scale degree are both lowered one half step.

It’s important to note here that the terms flat 3 and flat 5 don’t mean these notes will necessarily be flats. These notes are only the third and fifth notes occurring in the scale degree lowered a half step.

Therefore, if someone asks you to write down an F diminished triad, you first write the key signature for F, and then you write your triad on the staff, using F as your root and lowering the third and fifth intervals one half step.

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If you were to build an A flat diminished triad, you would go through the same process and come up with a triad that looks like the one below.

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Note that the perfect fifth of A flat major is an E flat — flatting the fifth makes it a double flat.