Dorothy Smith wrote:

>Would someone (or several people) take a few minutes to answer a
>performance question that has been puzzling me? Are there straightforward
>rules about how to play a triplet rhythm in one voice against a dotted
>rhythm in another? ...  I've read that in earlier music (through baroque)
>the two last notes of these figures should be played together.  However,
>one often finds in later music (for instance, Chopin's Polonaise-Fantaisie)
>that the score is clearly printed as if these notes are meant to be struck
>simultaneously.  If so, why does the composer write the dotted rhythm?--why
>not just triplets in each voice? If not, why is the printing consistently
>misleading? I suppose I should be able to sort this out by listening to
>performances, but those I have been able to hear never clearly answer this
>question for me.  Probably I am just being stupid--but, if so, that should
>make it a quick and easy job for someone to set me right!  ...

You are not being stupid at all, Dorothy.  Here's the skinny.  Pre-Haydn,
composers did not use the triplet as a rhythmic figure in simple duple
meter.  Therefore when you had a figure in compound meter(6/8, 9/8.12/8)
in which you wanted the accompanying figure to sound long-short long-short
etc against a three patterned melody, the dotted eighth sixteenth notation
was used.  It was standard practice to normalize the long-short figure to
match the three notes in the other voice as opposed to dividing the dotted
eight-sixteenth pattern into four sixteenth notes in value (whew).  Later,
by about Haydn's time, composers started using triple figures in simple
duple meter.  (2/4, 4/4) and thus it became clear that when they wrote a
Triplet in one voice and a dotted eighth-sixteenth note in the other,
that they meant it literally.

Kevin Sutton