In Sonnet 116, Shakespeare employs synecdoche in lines 1-2: "Let me not to the marriage of true minds / Admit impediments." Synecdoche is the use of a part of something to stand in for the whole thing. It isn't just people's minds that get married, it is the whole two individuals that wed. Therefore, "minds" stands in for the whole person. It's a somewhat ironic, or unexpected, choice because we typically think of romantic relationships as a joining of two hearts, if we use any part of the body to describe it. Irony occurs when reality differs from what we'd expect, and since we tend to associate the heart, not the mind, with love, such a choice is ironic.
Further, Shakespeare personifies "Love" as well as "Time," giving them human attributes and raising this story of love almost to the mythic since he ascribes intention and consciousness to intangible entities.
Shakespeare employs synecdoche again in lines 9-10: "Love's not Time's fool, though rosy lips and cheeks / Within his bending sickle's compass come." The sickle is a symbol of mortality (since we only have so much time before we are, figuratively, cut down by it), though time does not only claim our "rosy lips and cheeks"; again, it claims our whole selves. But this example of synecdoche allows Shakespeare to employ a visual image as well.
Perhaps the most striking figure of speech in the poem comes in line 9.
"Love's not Time's fool..."
This phrase expresses much of the poem's intention with a succinct and easily remembered turn of phrase, which makes it a figure of speech (as well as an example of figurative language).
Comparing love to a fool, the phrase is an example of personification - giving human qualities to a non-human concept. Love, of course, is not a person and is not subject to the failures of conscience or intelligence that humans are subject to. This is part of the point.
The poem suggests that true love is immovable. Love "alters not" with the passage of time. Rather, love is "an ever-fixed mark" that cannot be shaken by the vicissitudes of time. The timelessness or enduring qualities of love stands as the central theme and message of the poem.
To say that love is not time's fool then is to say that true love is stable, lasting and, in a sense, absolute.
Shakespeare uses many types of figurative language in "Sonnet 116," particularly an extended metaphor to relate the idea of unchanging love to nautical terms. In his opinion, love should be an "ever-fixed mark that looks on tempests and is never shaken;" a tempest is a violent squall, usually used to describe fast moving storms at sea (5).
The next line reinforces Shakespeare's nautical extended metaphor by comparing the surety of true love to a star that "every wandering bark," or lost ship, could use to navigate themselves home safeley (7). Through his use of extended metaphor, Shakespeare conveys the theme of steadfast love in the face of hardship.