A tercet is a group of three lines rhyming lines, usually in identical meter. Or so some dictionaries and glossaries say; like Paul Fussell, I would prefer to use "triplet" to designate a unit of three consecutive rhyming lines, and reserve "tercet" for a freestanding three-line unit of any kind. But the consensus seems to favor making "triplet" the generic term. Sometimes in the middle of a passage of neoclassical closed heroic couplets, however, a tercet will appear in which the third line is an alexandrine, or iambic hexameter.  Tercets may be in various meters. First below are some iambic tetrameter tercets:

                            A rose of perfect red, embossed
                            With silver sheens of crystal frost,
                            Yet warm, nor life nor fragrance lost.

                            High passion throbbing in a sphere
                            That Art hath wrought of diamond clear,
                            -- A great heart beating in a tear.

                            The listening soul is full of dreams
                            That shape the wondrous-varying themes
                            As cries of men or plash of streams.

                            Or noise of summer rain-drops round
                            That patter daintily a-ground
                            With hints of heaven in the sound.

                            Or noble wind-tones chanting free
                            Through morning-skies across the sea
                            Wild hymns to some strange majesty.

                            O, if one trope, clear-cut and keen,
                            May type the art of Song's best queen,
                            White-hot of soul, white-chaste of mien,

                            On Music's heart doth Nilsson dwell
                            As if a Swedish snow-flake fell
                            Into a glowing flower-bell.
                                                --Sidney Lanier
 

These are (as becomes more clear further on in the poem) trochaic octameter tercets:

                                       I.

                            Oh Galuppi, Baldassaro, this is very sad to find!
                            I can hardly misconceive you; it would prove me deaf and blind;
                            But although I take your meaning, 'tis with such a heavy mind!

                                    II.

                            Here you come with all your music, and here's all the good it brings.
                            What, they lived once thus at Venice where the merchants were the kings,
                            Where Saint Mark's is, where the Doges used to wed the sea with rings?
                                                                                        --Robert Browning

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