What is the structure of an Italian sonnet?

The structure of a typical Italian sonnet of the time included two parts that together formed a compact form of argument. First, the octave (two quatrains), forms the proposition, which describes a problem, or question, followed by a sestet (two tercets), which proposes a resolution. Typically, the ninth line initiates what is called the turn, or volta, which signals the move from proposition to resolution. Even in sonnets that don't strictly follow the problem/resolution structure, the ninth line still often marks a turn by signaling a change in the tone, mood, or stance of the poem.