A fairly straightforward verse form. It doesn't prescribe a set metre, but in English the iambic pentameter is often used. It consists of a series of three-line stanzas, or tercets, which are linked in that the middle line of a stanza provides the rhyme for the first and third line for the subsequent one. The poem closes with a single line (or occasionally a couplet) which rhymes with the middle line of the preceeding stanza. The rhyme scheme used is therefore: aba bcb cdc [etc] ded[e].

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     This is an example from Robert Frost. It is a variation from the standard form since it finishes with a rhymed couplet.

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Acquainted with the Night

                   I have been one acquainted with the night.
                   I have walked out in rain - and back in rain.
                   I have outwalked the furthest city light

                   I have looked down the saddest city lane.
                   I have passed by the watchman on his beat
                   And dropped my eyes, unwilling to explain.

                   I have stood still and stopped the sound of feet
                   When far away an interrupted cry
                   Came over houses from another street,

                   But not to call me back or say good-by;
                   and further still at an unearthly height
                   One luminary clock against the sky

                   Proclaimed the time was neither wrong nor right.
                   I have been one acquainted with the night.