What is the 'turn' in a sonnet, and where does it fall?

The turn, or volta, is the point where a sonnet swerves from one thought or mood to another, and where it falls depends on which kind of sonnet you are reading. A sonnet is a fixed poetic form of fourteen lines with a set rhyming scheme. In the typical Italian sonnet, the first eight lines, the octave, set out a problem or question, and the last six, the sestet, offer a resolution. The ninth line starts the turn that carries the poem from one to the other, and even in sonnets that do not follow this problem-and-answer plan, the ninth line often still marks a change of tone, mood or stance.

Shakespeare's sonnets are built differently: three quatrains, each of four lines, and a final couplet, rhymed ABAB CDCD EFEF GG. There the volta often comes at the end of the third quatrain, where the mood shifts and the poet turns the thought. But it is not a fixed address. Sonnet 18, "Shall I compare thee to a summer's day", has its volta beginning with the third quatrain: after noting that summer's qualities change and fade, the speaker says the young man will live forever in the lines of the poem, as long as it can be read.

The form also had a boom and a backlash. About four thousand sonnets were composed in England in the Elizabethan years, and much of it was conventional and repetitious. Shakespeare himself dismissed some of it in Sonnet 130, "My mistress' eyes are nothing like the sun."

And the word volta is younger than the habit. Many earlier critics who discussed a turn in a poem did not use it.