Below you will find a chart on the most often found and often misunderstood altered chords that we encounter in classical composition. When I was in college all the theory courses were taught using keyboard examples of chord progressions which of course was very helpful in understanding the voice-leading but never really clicked for me as a guitarist. The idea here is to present the most common altered chords in a context that adheres to the rules of voice-leading but is easily played on the guitar. I used four-part writing for the most part and found it to be surprisingly easy to play these on the guitar in first position.
I began with a simple diatonic progression I-IV-(I)-V-I and progressively added altered tones producing altered chords grouped into in three categories. The links will open examples from my analyses of the standard repertory.
1) Borrowed chords which borrow tones from the parallel minor scale.
2) Secondary dominant and secondary leading-tone chords.
3) Italian, German and French augmented sixth chords.
I included the Neapolitan sixth chord in the borrowed chord section since it is a chromatic variant of the minor iv chord. The flat second scale degree is actually borrowed from the Phrygian mode. This harmony is sometimes referred to as a Phrygian chord or Phrygian II.
I show two resolutions of the #iv⁰7 (vii⁰7/V) that are commonly found in the repertory. The first is the traditional resolution directly to the dominant (G). The second resolution is to the tonic six-four chord. Remember that the tonic six-four in this context functions as the dominant with a double appoggiatura. In this case we do have a common-tone (C) in the soprano, but since this C is actually part of a suspension figure resolving to B we do not strictly speaking have a common-tone diminished seventh chord here but just your basic vii⁰7/V.
One other note; I inverted the augmented sixth interval (Ab-F#) of the augmented sixth chords in order to better show its relationship to the subdominant. The active interval now forms a diminished third (F#-Ab). They both pull strongly in contrary motion to the dominant (G). Augmented sixth chords are often found in this "inversion".
Notice how all of these altered chords function as dominant preparation. In other words they usually precede the dominant (V) chord or the tonic six-four chord (fifth in the bass) that I have used in most of my examples.
Think of these altered chords as chromatic variants of the subdominant (IV) or its related supertonic (ii) chord since this is the harmony that usually prepares the dominant. I tried to show this relationship by preceding each altered chord with the diatonic or unaltered subdominant chord so you can easily see the voice-leading and which notes are altered.
Play these progressions many times in order to acquaint yourself with the sound of these harmonies and you will start to notice their occurrence in the classical repertory you perform. Some of the differences in the sound of the various harmonies are very subtle as with the vii⁰7/V and the Gr+6 for example.
All of these harmonies can be found in the writing of the composers that I have analyzed in my previous articles along with detailed explanations of their structure. Please look into some of these analyses for further explanation as well as how they are used in the compositions of the great composers for the guitar.