Yes, string trios are definitely a connoiseur's medium. Apart from
Beethoven's I am aware of one by Penderecki (which I've never heard), one
by Webern (the theory geek in me needs to mention that this is his first
12-tone composition for instruments alone), one by Dvorak (a Terzetto for
2 violins and viola), and one by Kodaly (a Serenade, also for 2 violins
and viola). Schubert also wrote two for the conventional combination (one
finished, one not), and, IIRC, Mendelssohn arranged some Bach for this
combination.
Aaron J. Rabushka
[log in to unmask]http://www.cowtown.net/users/arabushk/
