A terzetto is generally a composition for three voices.
Beyond one instance in Bach, and a few modern examples consisting of pieces not in sonata form, the tern has never been applied to instrumental music. It is obsolete, being superseded by Trio, which is the name given to music written for three instruments, and now includes vocal music as well.
A terzetto may be for any combination of three voices, whether for three trebles – as the unaccompanied Angels’ Trio in ‘Elijah,’ those of the three ladies and the three boys in ‘Die Zauberflote,’ the famous trio in ‘Il Matrimonio Segreto,’ and that for three florid sopranos in Spohr’s ‘Zemire und Azor’ – or for three male voices, like the canonic trio in the last named opera.
More frequent, naturally, are terzetti for mixed voices, the combinations being formed according to the exigencies of the situation.
There is nothing to be observed in the form of a terzetto different from that of any other vocal composition; but as regards harmony it should be noticed that when a bass voice is not included in the combination, the accompaniment usually supplies the bass (where 4 part harmony is required) and the three upper parts, taken by the voices, must be so contrived as to form a tolerable 3 part harmony themselves.