* Sadko 322-324—Mezzo-sopr. and Tenor.
As a general rule, writing for two voices is only successful when the progression of parts is clear, when discords are prepared by a common note, or are the outcome of conveniently separated movement and correctly resolved. Empty intervals of fourths and perfect fifths, elevenths and twelfths should be avoided on the strong beats of a bar, especially on notes of some value. If, however, one of the voices assumes a melodic character, the other forming the harmonic accompaniment in declamatory style, it is not absolutely necessary to avoid the intervals mentioned above.
Note. It is not within the scope of the present work to consider the writing of vocal parts in closer detail. This question must be left to the professor of free counterpoint. It remains to be noted that the human voice accompanied by the orchestra is always heard independently as something apart, something complete in itself. For this reason a composer may never rely on the orchestra to fill up an empty space or correct a fault in the handling of voices. All the rules of harmony and counterpoint, down to the last detail, must be applied to vocal writing, which is never dependent upon orchestral accompaniment.
Trios, quartets etc.
All that has been said regarding the relationship of voices in duet applies with equal force to the combination of three, four, five or more voices. An ensemble of several voices is seldom purely polyphonic; as a rule, although some parts move polyphonically, progression in thirds, sixths, tenths and thirteenths is used for the remainder. Declamation for some voices on notes forming the harmony is also possible. This variety of simultaneous movement of vocal parts renders the comprehension of the total effect less difficult for the ear, and sanctions the distribution of distinctive and suitable figures or tone colouring to certain voices with other figures or timbres which may be proceeding at the same time. The skilful arrangement of pauses and re-entries facilitates the understanding of the whole, and gives desirable prominence to detail.
Examples:
Snegourotchka 267—Trio, Finale to Act III.
The Tsar's Bride 116-118—Quartet in Act II.
""" 168-171—Sextet in Act III (cf. extract, [Ex. 283]).
Servilia 149-152—Quintet in Act III.