Any note may be a pitch note; for orchestras custom has settled upon a' in the treble clef, for organs and pianos in Great Britain c 2, and for modern brass instruments b flat'.
In the orchestras of his day this was perhaps the only safe proceeding for players unaccustomed to such responsibilities, and that may have been one of Beethoven's reasons for it.
Haydn's numerous examples of independent violoncello melodies are almost all either marked solo or written for such small orchestras that they would be played as solos.
Richard Strauss, in his edition of Berlioz's works on Instrumentation, paradoxically characterizes the classical orchestral style as that which was derived from chamber-music. Now it, is true that in Haydn's early days orchestras were small and generally private; and that the styles of orchestral and chamber music were not distinct; but surely nothing is clearer than that the whole history of the rise of classical chamber-music lies in its rapid differentiation from the coarse-grained orchestral style with which it began.
Such matters cannot be decided off-hand by the mere fact that tremolos are characteristic of orchestras: the question is whether in individual cases they have not a special character when played by single players.
Harmony is unknown, and orchestras, which include fiddles, flutes, drums and harmonicons, perform in unison.