I'm currently reading John Mortensen's wonderful book on keyboard improvisation. (See this link here.)

In the chapter on figuration preludes he states that the progression I - II4/2 - V6/5 - I was a popular "opener" or "page one" progression for such preludes. The most famous example being of course Bach's Prelude in C Major from WTC 1, BWV 846.

But why this particular progression? In particular, I wonder about the role of II4/2 here. Not by any stretch an uncommon chord in the late Baroque, but not particularly ubiquitous either.

Of course, the goal of any opening progression should be to establish the tonality firmly, but there must be dozens of progressions that accomplish this.

  • Was it as famous before the Bach Prelude, or did it really become popular afterwards? – Richard Jun 5 at 19:39
  • @Richard: Honestly, I don't know. I suppose Bach wasn't the very first to come with it. – Kim Fierens Jun 5 at 20:47
  • @Richard as far as I can tell there's no evidence that the Bach prelude was particularly widely known during the baroque period. – phoog Jun 5 at 21:08
  • Now I really want to get that book! – Michael Curtis Jun 5 at 22:15
  • @MichaelCurtis It does look good. I know you've cited Gjerdingen before, does he (or does Sanguinetti, etc.) ever mention this particular idiom? – Richard Jun 6 at 14:37

If you are talking about the part of the book on pages 6 and 7 (that's what I can see in the Amazon preview), the author gives V6/5 in the harmonic skeleton, but some of the examples hold a tonic pedal for all the chords. So I think we need to be flexible about that particular chord.

When looking at the Bach example in D minor I notice, if you hold a tonic pedal and then play in the treble, the scale in thirds descending from the tonic, you pass through the sort of prototype "opener" harmony.

For simplicity, transposed to C...

enter image description here

I'm not sure this is really an answer. It's more an observation.

Of course ii4/2 is then just the result of holding a pedal while the treble works through the motion that - despite the endless variety of figuration - can be viewed as a descent from the tonic by thirds.

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  • See example 2.2, he really does specify it with the Roman numerals I mentioned. Otherwise I like your explanation, which seems to hint that it's basically just a harmonic elaboration of a pedal point. Mortensen does mention the Quiescenza schema as another popular opening. – Kim Fierens Jun 5 at 23:15
  • @KimFierens, Yes, I missed it! I tried re-wording my answer accordingly. – Michael Curtis Jun 8 at 20:05

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