The third quatrain of Shakespeare's Sonnet 18 reads,

But thy eternal summer shall not fade
Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st.

The repetition of "not" and "nor" here creates a striking rhetorical effect, as of a speaker anticipating objections and eliminating them before they can even be raised. The first statement is clearly contentious and may be thought nonsensical until the poet explains his meaning in the final couplet. The beauty of the addressee is eternal and will never fade. In literal terms, this is impossible.

Before the reader can object however, the poet makes another very similar statement, reinforcing the first, then another, which is even more categorical and impossible. Not only will the addressee's beauty never fade, but they will defeat the universal menace of death.

The effect of all this rhetorical confidence is to build up and lend force to the final assertion in the rhyming couplet: that as long as anyone reads poetry at all, this sonnet will preserve the speaker in perfect beauty. The chain of negatives in the third quatrain, with the poet making ever more confident assertions that the addressee is exempted from the common fate of mortals, render the concluding couplet even more striking as the triumphant peroration of a perfectly constructed argument, as well as being the climax of the poem.