The poem’s three stanzas share the same rhyme scheme: AABBCCDDD. All the rhymes in the poem are exact and take a so-called masculine form, meaning they occur on the final stressed syllable of each line. The only rhyme that’s even remotely imprecise is the poem’s first, where the sibilant S of “this” (line 1) differs slightly from the voiced S of “is” (line 2). Otherwise, the rhymes in the poem are all exact. The dominance of exact rhyme isn’t surprising for a poem from the seventeenth century, when the use of inexact or slant rhyme wasn’t a common practice. Yet the exactness of the rhymes in “The Flea” also has a significance beyond mere convention. Reconsider the poem’s rhyme scheme: AABBCCDDD. Note how Donne has broken each stanza into a series of rhyming couplets, except for the triple rhyme that appears in the final three lines. The couplet form stands in contrast to the way each line alternates between tetrameter and pentameter. Thus, despite having different lengths, the lines in each couplet nonetheless rhyme perfectly. These shared rhymes indicate that the speaker and his mistress share a similar—if still not perfectly commensurate—interest in having sex.