Terza rima

                         aba, bab, cdc, ded, etc. 

In other words one rhyme-sound is used for the first and third line of each stanza and a new rhyme introduced for the second line, this new rhyme, in turn, being used for the first and third lines of the subsequent stanza. Usually the meter is iambic pentameter. The opening of Shelley's "Ode to the West Wind," which is written in terza rima, illustrates it:

     O wild West Wind, thou breath of Autumn's being,          a
     Thou, from whose unseen presence the leaves dead          b
     Are driven, like ghosts from an enchanter fleeing         a

     Yellow, and black, and pale, and hectic red.              b
     Pestilence-stricken multitudes: O thou,                   c
     Who chariotest to their dark wintry bed . . .  b 

The terza rima has been popular with English poets, being used by Milton, Shelley, and Byron, among many others. With variations in meter and the use of imperfect rhymes, it has been widely used by contemporary poets, particularly Archibald MacLeish, W. H. Auden, William Carlos Williams (in "Yachts"), and T. S. Eliot.

Williams's "Yachts" is indebted to the terza rima of Dante's Divine Comedy, which influenced Shelley's use of the form as well.

The American poet Robert Pinsky has retranslated Dante's Inferno, retaining the terza rima (no easy trick). Here is part of Rachel Jacoff's review of Pinsky's effort:

With Robert Pinsky's Inferno, the tradition of poets rediscovering Dante has a new and splendid exemplar. A poetic reappropriation of Dante had been the goal of Daniel Halperin's earlier project of farming out the Inferno to twenty different contemporary poets, each of whom was to translate one or two Cantos. It was translating Cantos 20 and 28 for that volume that got Pinsky hooked on the challenge of an English- speaking Dante. In the Introduction to the Halperin book James Merrill wrote of the potential for twenty further translations, but thus far Pinsky's is the only one.

Pinsky has many virtues as a translator. His ear for tone is much sounder than that of other recent translators, as is his ear for spoken English. His translation rarely produces the discomfort I feel reading aloud in a classroom from Singleton or Mandelbaum. Pinsky's taut rendering gives it a robust muscularity. There is neither archaism nor the awkward padding characteristic of many verse translations, which fill out lines for purposes of rhyme.

Rhyme is a central concern for any translator of the Commedia because of the importance of terza rima, a rhyme scheme of great narrative momentum and thematic suggestibility. In terza rima the first and third lines of each tercet rhyme with each other and with the central line in the antecedent terzina (aba bcb cdc and so on), producing the effect of two steps forward and one step back. With its seamless blend of forward motion and backward glance, the verse form has the nearly compulsive energy of waltz rhythm. Since rhyme is achieved so much more easily in Italian, the rhymes feel neither forced nor exaggeratedly emphatic. Because English is a language with greater lexical resources but far less capacity for rhyme, rhyming on the scale demanded by terza rima feels more like chiming, and is often obtrusive or comic. For this reason, some translators have modified the verse form (rhyming only the first and third lines of the tercet), or allowed themselves great leeway with inexact rhymes, or rhymed sporadically. Pinsky opts for consonantal or slant rhyme as his basic scaffold to avoid the negative potential of strong rhymes.