General observations.
The art of orchestration demands a beautiful and well-balanced distribution of chords forming the harmonic texture. Moreover, transparence, accuracy and purity in the movement of each part are essential conditions if satisfactory resonance is to be obtained. No perfection in resonance can accrue from faulty progression of parts.
Note. There are people who consider orchestration simply as the art of selecting instruments and tone qualities, believing that if an orchestral score does not sound well, it is entirely due to the choice of instruments and timbres. But unsatisfactory resonance is often solely the outcome of faulty handling of parts, and such a composition will continue to sound badly whatever choice of instruments is made. So, on the other hand, it often happens that a passage in which the chords are properly distributed, and the progression of parts correctly handled, will sound equally well if played by strings, wood-wind or brass.
The composer should picture to himself the exact harmonic formation of the piece he intends to orchestrate. If, in his rough sketch, there exist any uncertainly as to the number or movement of harmonic parts, he is advised to settle this at once. It is likewise essential for him to form a clear idea as to the construction and musical elements of the piece, and to realise the exact nature and limitations of the themes, phrases and ideas he is going to employ. Every transition from one order of harmonic writing to another, from four-part harmony to three, or from five-part harmony to unison etc., must coincide with the introduction of a new idea, a fresh theme or phrase; otherwise the orchestrator will encounter many unforeseen and insurmountable difficulties. For example, if, during a passage written in four parts a chord in five-part harmony is introduced, a fresh instrument must needs be added to play this particular fifth part, and this addition may easily damage the resonance of the chord in question, and render the resolution of a discord or the correct progression of parts impossible.