Dante did not set out to be a great poet. Like many people living in
the Italian city-states during the late Middle Ages, his passion was
politics. And politics during Dante's time was nothing less than a blood
sport. The political party that he belonged to had split into two
competing factions, the "Blacks" and the "Whites" (there are no racial
overtones here--this simply refers to their symbolic colors). Dante was
a prominent government official of the White party, and, in 1302, he was
sent on an ambassadorial mission to Rome. While he was there, the Black
party overthrew the White party and Dante was placed under a death
penalty if he ever returned, so he lived the rest of his life in exile.
Unable to practice his chosen vocation, he turned to writing and
produced, among other works, The Divine Comedy." (taken from the
website:
http://courses.shepherd.edu/fall9900/maustin/engl208/dante/module15.htm)


In my opinion Terza rima is the most demanding rhyme and structure
construct that is used in English poetry. Many fine poets have attempted
to use it, none have really succeeded. Dante wrote the poem in Italian
(a radical approach at the time, as almost everything was still being
written in Latin) and terza rima was relatively easy to achieve.
(Italian words can possess only seven vowel sounds  (they are unmodified
by the adjoining consonant) while English permits fifty-two varieties of
sound  (precisely because they are subtly or not-so-subtly modified by
those same consonants)). So, as you can see, such is not the case in
English, and translators have had big problems doing justice to this
great work. (from Bill Massey's website) The problem with using the
terza rima in English is that it is extremely demanding, since each
rhyming ending must be represented not two but three separate times.
Whereas Italian, with its abundance of vowel endings, is a language rich
in rhyming possibilities, English, having a more complex and varied
inventory of sounds, is relatively rhyme-poor." (taken from Bruce
McMenomy)


The 'inside out' movement provides powerful narrative impetus.  Each
section of terza rima is closed with a couplet.  These may be for any
line length, but tend to break up into distinct line segments if there
are more than five beats to the line.  However, though English is not so
rhyme-poor as the cliché would have it, terza rima in English tasks the
poet to use every trick in the book to get it going, including near and
off rhyme, as well as assonance and consonance."
(quoted from the website: http://www.n2hos.com/acm/prospart4.html )