Here are the first few bars of the Lacrimosa in Mozart's autograph:
|Click to enlarge|
|Click to enlarge|
For those of you who are harmony geeks like myself, I'll just sort out what he is doing here. First, going through the condensed version. The key is D minor and the first chord is the tonic, D minor. Then a chord that is a dominant 7th in last inversion of the subdominant. Then the subdominant--in major. The next chord is the very fancy German augmented 6th chord which leads to the dominant with a 6/4 suspension.
Now let me explain what I just said! There are three important chords in every key. I say "three important" even though there are actually seven possible chords in every key and each one may be presented in different ways. You can also add extra notes like 7ths to each chord. But the other chords are related to the three important ones. Basically it boils down to the chord of rest and resolution, the tonic, the chord of tension, the dominant and chords that precede or prepare the dominant, often to add even more tension. The basic progression in the reduction above is tonic, subdominant, dominant.
Believe it or not, this is the same chord progression as the one in "Twist and Shout": tonic, subdominant, dominant, or D, G, A. The difference is that here we are in D minor instead of D major. Another difference is that the subdominant and dominant, the G and A chords, are each preceded by a chord that sets them up. The subdominant is preceded by its dominant--a neat trick that composers have been using for a long time, and the dominant is preceded by that fancy augmented 6th chord. The augmented 6th is fancy because it moves to the dominant with extreme tension. It has a leading tone up to the 5th of the dominant and a leading tone down to the root of the dominant. What is very neat about it is that its origins are really very old--it goes back to the Phrygian cadence of modal harmony. To boil it down a bit, these extra chords that are added to the simple tonic, subdominant, dominant progression come about through voice-leading. The old principles that the monks discovered when they were inventing counterpoint a thousand years ago, are still being used.
Now let's hear this music. First, "Twist and Shout":
Three chords all the way through... Then Mozart:
The passage I have been talking about is from the 25 second to the 50 second mark.
Amazing how pieces of music as utterly different as these two turn out to have similar roots!