Diminished Chord - Uses the symbol "o" or "dim" and is formed by playing the first note (root) + flatted third (b3) + flatted fifth (b5) of a major scale. When you add another minor third (b3) 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 tetrad 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 - Uses the symbol "+" or "aug." and is formed by playing the first note (root) + third note + sharped fifth (#5th) of a major scale.
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#