You might have guessed by the name of this form that we're witnessing the very beginning of this special type of poetry in "Chiare, fresche et dolci acque." Petrarch is not really inventing anything new when he uses this form. Nope, he gets the idea from the Portuguese canso and the Provençal troubadours singing their chansons.
But, like any good innovator, Petrarch takes what he knows and riffs on it, altering things to suit his needs. The structure of this poem is pretty straightforward: we've got five strophes (i.e., stanzas) with 13 lines each, and a congedo (farewell) or envoi of three lines at the end.
Each stanza has an internal division into two parts: the fronte and sirma. The fronte, which is composed of two strophes, has a rhyme scheme of ABCABC (where each letter stands for that line's end rhyme). The sirma, which is the tail, usually has some variation of CDEEDFF.
Okay, don't panic! Examples? We've got plenty. Here's one, taken from the second stanza:
S' egli é pur mio destino,
e'l cielo in ciò s'adopra,
ch'Amor quest' occhi lagrimando chiuda,
qualche grazia il meschino
corpo fra voi ricopra
e torni l'alma al proprio albergo ignuda;
la morte fia men cruda
se questa spene porto
a quel dubbioso passo,
ché lo spirito lasso
non poria mai in più riposata porto
né in più tranquilla fossa
fuggir la carne travagliata et l'ossa. (14-26)
We know what you're thinking: "How did you come up with this, Petrarch? Oh yeah, and why, why, why?" Well, if you go back to those Provençal troubadours, you'll get your answer. Because this type of poem was meant for performance (to be sung), it had to be melodic in nature (it's a lyric,after all).
The change from fronte to sirma shows an evolution of the rhyme scheme: ABCABC gets left behind and the verse switches up to a more complex and (mostly) new set of rhymes (CDEEDFF). So the change up in the rhyme scheme allows for a progression of the melody of the poem. No one wants a song that never changes, right?
Meter: Hendecasyllables and Settenari
We know, we know: hendewho and settewhati? Stick with us, Shmoopers. All will be revealed. You see, Petrarch loved the poets of classical antiquity, so he used metrical patterns from Latin and Greek poetry (though he couldn't read Greek). Both Canzone126, and its partner poem Canzone125, have hendecasyllables and settenari (seven-syllable lines with two or three main stresses—and one of them always on the sixth syllable). To be precise, there are nine settenari (short lines) and four hendecasyllables (long lines) in each stanza.
Now, this all very well and good. But before you knock yourself out trying to count syllables in a foreign tongue, a word of caution: the lines can have a few extra syllables in them and still make it under the radar. It's easier to show than tell on this one. Take the sirma of stanza (or strophe) 2, with syllable counts in parentheses:
La morte fia men cruda (8)
se questa spene porto (7)
a quel dubbioso passo: (8)
ché lo spirito lasso (7)
non poria mai in più riposato porto (13)
né in più tranquilla fossa (8)
fuggir la carne travagliata et l'ossa. (12) (20-26)
There's not an 11-syllable line in sight here, and two of the 7-syllable lines are actually... carry the one… eight syllables. Don't sweat it. Focus on the pattern of stress. The long lines (i.e., the alleged 11-syllable lines) must have a stress on the tenth syllable. The short lines (i.e. the supposed 7-syllable lines) must have a stress on the sixth syllable. Note that they do, more or less.
It's crucial to hear the poem read aloud in order to catch these stresses and to hear the effect they have on the sound of the poem. Take a listen to Moro Silo narrating Petrarch's Canzone126 and ride the melodic waves of Petrarch's medieval Italian.