Seven chords, and three functions.
Here are the diatonic chords in a-minor:
- Am, Tonic
- Bm-5, Supertonic
- C+, Mediant
- Dm, Subdominant
- E, Dominant
- F, Submediant
- G♯m-5, Leading-tone
There are three functions a chord can have in a song: Tonic Function, Subdominant function, and Dominant Function.
A tonic function occurs when a chord controls the "stability" of a musical piece. A tonic, submediant, or a mediant chord can have a tonic function.
A subdominant function occurs when a chord works itself on the "contrast" from the tonic-function chords. They can either come before a tonic-function or a dominant-function chord. A supertonic or a subdominant chord can have a subdominant function.
A dominant function occurs when a chord works itself on the "tension". It will give a tension that gives us an idea that it will resolve to a tonic-function chord. The dominant and the leading-tone chord can have this function.
In a-minor, here is how the chords function themselves:
Tonic function: (Stability)
- Am, tonic
- C+, mediant
- F, submediant
Subdominant function: (Contrast)
- Bm-5, supertonic
- Dm, subdominant
Dominant function: (Tension)
- E, dominant
- G♯m-5, leading-tone
However, the F chord in the case written above functions as a subdominant, because it comes before a chord that functions as a dominant. The dominant-function chord is G, but it still acts the same as E and G♯m-5 chords, as it leads back to the tonic.
Also, there is a type of cadence called a "plagal" cadence. It is usually IV - I (iv - i), but sometimes it can be altered into ii6 - I (iio6 - i), ii6/5 - I (iiø 6/5 - i), or vi - I (VI - i). In these cases, the supertonic, the subdominant, and the submediant chords all have the subdominant function.
Also, the first movement of Beethoven's Piano sonata No. 32 ended with a viio 6/5 - I cadence. In this case, the viio 6/5 chord functions the same way as a V chord, and has dominant function, which pushes us to the tonic chord. A dominant-tonic cadence is an authentic cadence (but only if the tonic is REALLY a tonic chord).