In the collection of 154 sonnets by William Shakespeare published in 1609, Sonnet 99 is unique for two reasons that are possibly related: it is the only sonnet to consist of 15 lines instead of the usual 14, and it is the only sonnet that leans directly on a known source and can therefore be said to be a more or less direct reworking of an existing piece by another poet, rather than presenting a mere variation on a well-worn theme. The theme itself though is familiar from both classical and Renaissance poetry, but Shakespeare, as we would probably expect by now, manages to furnish his poem with one twist in particular that suggests he may be engaging in more than just a standard rhetorical exercise of imitatio.