The subdominant area actually has two functions. Apart from preparing the dominant it is typically found in the coda of a sonata or other piece. After a great deal of fuss and energy spent going from the tonic to the dominant, a lot of pieces have a section near the end where the tension is relaxed by having a passage in the subdominant.
But the main subdominant function is to get us to the dominant in interesting ways. The tonic is a fixed, known quantity and so is the dominant. But the subdominant has proven to be a wonderfully flexible area that has attracted much of the harmonic ingenuity composers have come up with. The subdominant function or area can be the subdominant chord or the supertonic chord as they share two notes. But it can also be something more exotic, something that goes to the dominant in a more intense way. Two of the most interesting harmonies that serve this function are the Neapolitan sixth and the augmented sixth chords.
The Neapolitan sixth chord is a major chord built on the flattened supertonic. In harmonic analysis it is written as either bII6 or N6. In C major it would be a D flat chord. It is usually found in first inversion, which is why it is called a sixth chord. In C it would be spelled F A flat D flat. How is this a subdominant function? For one thing, the bass note is F, which is the subdominant. Also, it has the subdominant function as it characteristically moves to the dominant: it is a "pre-dominant" harmony. The Neapolitan sixth is most often found in the minor mode and it gives a momentary "Phrygian" feel to the harmony because of the flat second degree. What makes it different is that the flat second degree, rather than resolving down to the tonic, as it would if it were truly Phrygian, it goes instead to the leading tone (part of the dominant chord) and then to the tonic. Here is how it looks in D minor, first how it moves to the dominant and then in a full cadence.
The effect of the Neapolitan is to greatly intensify a cadence. Here is how it sounds in an example from Mendelssohn. This is from the Songs Without Words, op 102, no 4. The key is G minor, so the Neapolitan Sixth is spelled C E flat A flat. It occurs in the second half of the fifth measure, at the 17 second mark in this clip:
Here is the score:
The progression is quite typical of the Neapolitan: it goes to the dominant which cadences to the tonic. Bach, of course, created a much more complex and interesting context for the Neapolitan, using it in a passage that first suggests a deceptive cadence, then moves to the Neapolitan and finally a V to I, but not a perfect authentic cadence as both chords are in inversion. Pretty fancy!
I quoted this in a whole post about the deceptive cadence.
There is a lot more interesting stuff that can go on in the subdominant area, but I will save it for another post. What I am calling the "subdominant area" is that whole group of chords, including the subdominant, the supertonic and the Neapolitan, that all serve a subdominant function and lead to the dominant.
The effect of the Neapolitan is to greatly intensify a cadence. Here is how it sounds in an example from Mendelssohn. This is from the Songs Without Words, op 102, no 4. The key is G minor, so the Neapolitan Sixth is spelled C E flat A flat. It occurs in the second half of the fifth measure, at the 17 second mark in this clip:
Here is the score:
The progression is quite typical of the Neapolitan: it goes to the dominant which cadences to the tonic. Bach, of course, created a much more complex and interesting context for the Neapolitan, using it in a passage that first suggests a deceptive cadence, then moves to the Neapolitan and finally a V to I, but not a perfect authentic cadence as both chords are in inversion. Pretty fancy!
I quoted this in a whole post about the deceptive cadence.
There is a lot more interesting stuff that can go on in the subdominant area, but I will save it for another post. What I am calling the "subdominant area" is that whole group of chords, including the subdominant, the supertonic and the Neapolitan, that all serve a subdominant function and lead to the dominant.