The Petrarchan Mode in literature refers to the particular structure of Petrarchan Sonnets. Sonnets are 14 line poems with a very rigorous structure, and there are two main types of Sonnets: Shakespearean sonnets and Petrarchan (or Italian) sonnets.
Petrarchan sonnets are sonnets consisting of an octave (an eight line block of verse) and a sestet (a six line block of verse). The octave can be broken down into two quatrains and the sestet can be broken down into two tercets. Generally the octave presents an idea that is contrasted by the concluding sestet. Consider the following example:
In what bright realm, what sphere of radiant thought
Did Nature find the model whence she drew
That delicate dazzling image where we view
Here on this earth what she in heaven wrought?
What fountain-haunting nymph, what dryad, sought
In groves, such golden tresses ever threw
Upon the gust? What heart such virtues knew?—
Though her chief virtue with my death is frought.
He looks in vain for heavenly beauty, he
Who never looked upon her perfect eyes,
The vivid blue orbs turning brilliantly –
He does not know how Love yields and denies;
He only knows, who knows how sweetly she
Can talk and laugh, the sweetness of her sighs.
Sonnet 159 by Petrarch
The poem taken as a complete unit is writen to extol the virtues of love, but there is a definite shift of focus at the transition between the octave ans sestet. The octave poses a number of questions in respect and admiration of the object of their affection, the sestet is a lament to the man who has not experienced the joys of love in his life.