Like many of Shakespeare's sonnets, Sonnet 18 follows a 4/4/4/2 division. That is to say, the first four lines (or quatrain) are grouped together, then the next four, and then the next four, followed by the last two lines (or couplet) of the work that sum up and/or reflect on the poem to that point. Sonnet 18's first two quatrains are closely linked: both talk about the beloved's beauty being more constant than nature's beauty, which changes: the sun gets hidden, good weather turns bad, etc. The next four say that this won't happen to the beloved, though, ending with the reason why: this poem. The last two lines then state this specifically.