What Are Diminished and Augmented Triads?

triad C augmented
Triad C augmented. Von Tobias Langhammer - I created it / selbst erstellt, CC BY-SA 3.0, Link

Triads are chords formed by three notes played together that consist of the root note, the third, and the fifth of a scale. In a triad, the root note is at the bottom with the third and fifth stacked above. Diminished and augmented chords are two types of triads.

Augmented triads have an unusual, mysterious sound, while diminished chords have an unsettling, dissonant sound. The other two types of triads are major and minor.

Diminished Chord

In a diminished triad, the middle and top two notes of the chord—called the third and the fifth — are flattened (lowered a half step). It is indicated by the symbol "o" or "dim." For example, the G triad based on a major scale is formed by playing G (the root note), B (the third note), and D (the fifth note). A diminished G triad chord, therefore, consists of  G, B flat, and D flat. 

When you add another minor third to a diminished chord, it becomes a tetrad, or a four-note chord. The symbol used for this is "o7." Two commonly used types of tetrads are the dominant 7th (7) and the major 7th (maj7) chords.

Here are the diminished chords in different keys:

C dim = C - Eb - Gb

G dim = G - Bb - Db

D dim = D - F - Ab

A dim = A - C - Eb

E dim = E - G - Bb

B dim = B - D - F

F# dim = F# - A - C

Gb dim = Gb - A - C

Db dim = Db - E - G

C# dim = C# - E - G

Ab dim = Ab - B - D

Eb dim = Eb - Gb - A

Bb dim = Bb - Db - E

F dim = F - Ab - B

Augmented Chord 

In an augmented triad, the fifth or top of the three notes of the chord is sharpened (raised half a step). It is indicated by the symbol "+" or "aug." For example, the C triad in a major scale is formed by playing C (the root note), E (the third note), and G (the fifth note).

To create an augmented C triad chord, you would play a G sharp, rather than a G.

Here are the augmented chords in different keys:

C aug = C - E - G#

G aug = G - B - D#

D aug = D - F# - A#

A aug = A - C# - F

E aug = E - G# - C

B aug = B - D# - G

F# aug = F# - A# - D

Gb aug = Gb - Bb - D

Db aug = Db - F - A

C# aug = C# - E# (or F) - A

Ab aug = Ab - C - E

Eb aug = Eb - G - B

Bb aug = Bb - D - F#

F aug = F - A - C#