More on the Sonnet

In An Exaltation of Forms: Contemporary Poets Celebrate the Diversity of Their Art, Marilyn Hacker notes about the Italian sonnet "that all these versions of the sonnet almost predicate a poem whose "argument" divides into two parts, a premise set out in the octave (first eight lines), with the sestet contradicting it, modifying it, or giving a concrete proof."

The volta, or turn, seems to be the essential element that makes a sonnet a sonnet. Meter might vary, a couple lines might be added or subtracted, rhyming might be slant or varied or even non-existent, but most poet-critics suggest the turn is a requirement.

Hacker also says (specifically about sequences of sonnets):

Its Italian form is very like a mixture of the two most flexible and utilitarian "blocks" of verse narrative: the quatrain and terza rima.

And she summarizes the sonnet as (in regards to its origins and contemporary applications):

a poem in "popular" language that could be read or written by anyone (not only clerics and scholars) and that incited its writers to fresh examination of their evolving languages' interactions with the human world.