STRING QUARTETS APART, most new scores call for unique instrumental combinations. Composers generally select their combinations very carefully, taking into consideration the variety and balance of colors, the characteristic sonority of the group as a whole, and, above all, the richness of contrapuntal possibilities; the question of practicability on a mixed program is at most secondary. In a new sense, the choice of instruments has itself become an integral part of these works: composers seldom write for the same group twice and seldom adopt each other's combinations unmodified. Perhaps the term chamber music is obsolete; the real home of these works is not the rare, poorly attended, and very special concert for which alone they are suited, but the private tape or public disk recording. Morton Subotnick's Serenade No. I is a case in point. An eight-minute work, it requires six performers: flute, clarinet in Bb, vibraphone, mandolin, cello, and piano. An air reed, a single reed, a tuned percussion instrument, a plucked string, a bowed string, and a struck stringcertainly a vivid and transparent collection, at once problematic and musically suggestive. The mandolin part is comparatively easy: there are no multiple stops or fast passages, and any violinist who can learn to make an even tremolo should be able to play it. The other parts have a few awkward places, but a high level of virtuosity is not essential; what is essential is a high level of togetherness, a tight ensemble. The Serenade is in three movements. This division is indicated in the