Sonnet 18 is one of the best-known of the 154 sonnets written by English poet and playwright William Shakespeare.

In poetry, a couplet is a pair of successive lines that rhyme and have the same metre. A couplet may be formal (closed) or run-on (open). In a formal (closed) couplet, each of the two lines is end-stopped, implying that there is a grammatical pause at the end of a line of verse. In a run-on (open) couplet, the meaning of the first line continues to the second.