METRICAL STRUCTURE.--Each of the three parts, or cantiche, is divided into cantos: the _Inferno_ into thirty-four, the _Purgatorio_ into thirty-three, the _Paradiso_ into thirty-three--thus making up a hundred cantos, the square of the perfect number. Each canto is composed of from one hundred and fifteen to one hundred and sixty lines, forming thirty-eight to fifty-three terzine, a continuous measure of three hendecasyllabic lines, woven together by the rhymes of the middle lines, with an extra line rhyming with the second line of the last terzina to close the canto: ABA, BCB, CDC, DED ... XYX, YZY, Z. The normal hendecasyllabic line is the _endecasillabo piano_, in which the rhyme has the accent upon the penultimate syllable (_rima piana_, trochaic ending). Occasionally, but rarely, we find the _endecasillabo sdrucciolo_, with the rhyme accentuated on the antepenultimate syllable (_rima sdrucciola_, dactylic ending), or the _endecasillabo tronco_, with the accent on the final syllable (_rima tronca_). Italian prosody regards both these latter forms (which appear to have twelve and ten syllables respectively) as lines of eleven syllables.[29] The _terza rima_ seems to be derived from the _serventese incatenato_ (“linked serventese”), one of the rather numerous forms of the Italian _serventese_ or _sermontese_, a species of poem introduced from Provence in the first half of the thirteenth century. The Provençal _sirventes_ was a serviceable composition employed mainly for satirical, political, and ethical purposes, in contrast with the more stately and “tragical” canzone of love. Although the Italians extended its range of subject and developed its metres, no one before Dante had used it for a great poem or had transfigured it into this superb new measure, at once lyrical and epical. In his hand, indeed, “the thing became a trumpet,” sounding from earth to heaven, to call the dead to judgment. SOURCES.--The earlier mediaeval visions of the spirit world, of which the most famous are Irish in origin, bear the same relation, in a much slighter degree, to the spiritual content of the _Commedia_ as the Provençal _sirventes_ does to its metrical form. Even if Dante was acquainted with them (and there are episodes occasionally in the poem which recall the vision of Tundal or Tnuthgal), he was absolutely justified in asserting, in _Purgatorio_ xvi., that God willed that he should see His court “by method wholly out of modern use”: Per modo tutto fuor del moderno uso.