The use of four different timbres in close four-part harmony is to be avoided, as the respective registers will not correspond:
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Note. In Mozart and Salieri, which is only scored for 1 Fl., 1 Ob., 1 Cl. and 1 Fag., wood-wind chords in four-part harmony are of necessity devoted to these four different timbres.
The same rules apply to writing in three-part harmony, which is the most customary form when it is a question of establishing a harmonic basis, the lowest register of which is entrusted to another group of instruments (strings arco or pizz., for example). Chords in three-part harmony are generally given to two instruments of one timbre and a third instrument of another, but never to three different timbres. Overlaying of parts is the best course to adopt:
The use of crossing and enclosure of parts (which in a way amount to the same thing) must depend on the manner of their progression: