My name is Manuel Diewald and I am a Sydney-based guitar teacher in West Pennant Hills with a B.Mus. (Hons) (First) in Music Pedagogy from The University of New South Wales (UNSW).
You have probably heard of a Major and minor chord by now, if not you can read my previous blog called 'What is a Major and Minor Chord?' as it will help set the foundation for understanding what a diminished and augmented chord is.
Diminished chord: A diminished chord is based on a minor chord though we need to alter the 5th degree of the minor chord. If we take a quick look at the interval formula for a minor chord being minor 3rd and Major 3rd we simply alter the Major 3rd by turning it into a minor 3rd which creates a diminished chord. Therefore, a diminished chords interval formula is minor 3rd and minor 3rd. To make this easier to understand simply take a minor chord and flatten (by flattening a note you are moving that note down in pitch) the 5th degree by a semitone. For example, if we take an A minor chord A - C - E we need to flatten the 5th degree by a semitone which means our E needs to be an Eb. Now if we take this triad A C - Eb we now have a diminished chord. Try finding those notes on the guitar and hear what that sounds like. Diminished chords are given a circle symbol next to the note name which you can see in the image below.
Augmented chord: To create an augmented chord we need to take a similar approach though this time we are using the Major chord as our foundation and rather than flattening the 5th degree by a semitone we are raising (by raising a note you are moving that note up in pitch) the 5th by a semitone. For example, let's take a C major chord. The interval formula for a Major chord is Major 3rd and minor 3rd which means if we start on the note C we have C - E - G. To create an augmented chord we need to change the minor interval into a Major interval which gives us a Major 3rd and Major 3rd creating the interval formula for an augmented chord. For example, if we take our C Major chord we simply need to raise the 5th degree by a semitone which gives us C - E - G#. Try finding those notes on the guitar and hear how they sound. Augmented chords are given a plus symbol next to the note name which you can see in the image below.
Now try and take a C major chord, turn it into a minor chord and then turn it into a diminished chord. Do the same for a minor chord. Turn it into a Major chord and then into an augmented chord. They sound quite extreme in comparison to the Major and minor and you can probably hear now why they are used sparingly and not as commonly as Major and minor chords.