Consider following excerpt from Chopin's Minute waltz in D flat major.
The last bar is the chord of D flat major (tonic).
What is the chord of the penultimate bar? Why is there an A flat and A natural at the same time?
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The basic harmony of the penultimate bar is
Ab7 -- that is, the dominant seventh (
V7) chord of
Db major. This is seen most clearly in the left hand plus the final two right-hand notes.
Chopin could have written the final three measures this way:
X: 0 T: "Minute" Waltz Reduction #1 M: 3/4 L: 1/8 K: Db V:RH V:LH clef=bass [V:RH] y fedcBA | G4 C2 | D2 z2 |] [V:LH] y A,,2 [F,A,D]2 z2 | A,,2 [G,A,]2 [G,A,]2 | D,2 [A,F]2 |]
A on the downbeat is an accented chromatic tone leading the ear toward the
Bb on beat 2. Consider that Chopin might have written this:
X: 0 T: "Minute" Waltz Reduction #2 M: 3/4 L: 1/8 K: Db V:RH V:LH clef=bass [V:RH] y fedcBA | =A2 B2 C'2 | D'2 z2 |] [V:LH] y A,,2 [F,A,D]2 z2 | A,,2 [G,A,]2 [G,A,]2 | D,2 [A,F]2 |]
But, being Chopin, he made it much more interesting.
For additional understanding of the measure, take a look at Wikipedia's page on Nonchord Tones.
The main harmony of the penultimate bar is Ab7 as shown in the bass clef; it's an Ab7 with the third and fifth missing. The right hand in the treble clef plays what seems to be a bunch of non-chord tones (except for the Gb and C in the last notes). If we listen to the effect, this set of notes sounds a bit like an F chord (A, C, F played) then a Bb7 (Bb, F, Ab)., eb (Bb, Gb), Ab7(C, Gb, C).The two C's provide the third for the Ab7 being played in the bass clef and give a half-step approach to the Db in the final measure.
It's a decorated (rather cleverly) Ab7 chord. Chopin also (in one of the polonaises) has ornaments that exhibit a V-I movement (as secondary dominants) though similar constructs by other composers just use note clusters.
My first itch was to called this an Ab altered chord with a b9, but that would have been wrong (because the natural nine also appears)! So I would give two possible explanations:
The most common explanation would be to see this as a circling double approach of Bb: you first play a note below (A), then one above (C) and then the target note in the middle (Bb). You could also play a diatonic approach (Ab C Bb), but that would repeat the last note of the previous melody line. So opting for a chromatic approach here makes the melody more fluent. This would be a more melodic (horizontal) explanation.
Another phenomenon this reminds me of is to approach a chord by preceeding it with a diminished chord a half step below. This often happens against the original harmony (keeping the timing and the bass note) and is quickly resolved afterwards (Adim/Ab Ab). This would be more of a harmonic (vertical) explanation.
In music theory it is important to keep in mind that there are always multiple correct explanations of the same phenomenon. Pick the one which suits you best in the current situation.