Iambic tetrameter describes a particular rhythm in each line. The rhythm is measured in feet, small groups of syllables. Iambic describes the type of foot, and tetrameter tells us that there are four feet in each line. An iambic foot consists of one unstressed syllable followed by a stressed syllable. An unstressed syllable is marked by a breve ( ˘ ), and a stressed syllable is marked with a forward slash ( / ). The first line would be marked as follows:
˘ / ˘ / ˘ / ˘ /
But could / youth last, / and love / still breed,
Each combination of an unstressed and a stressed syllable equals one foot. Putting a short vertical line between each makes it easier to see how many feet there are in a line.
Rhyme scheme refers to the pattern of rhyme between lines of a poem. Letters, starting with (a), indicate which lines rhyme. For these lines, we are referring to the words at the end of each line that rhyme. Look at the last word of each line and see what other line that word rhymes with. "Breed" rhymes with need, so those two lines are marked with (a). The last two lines rhyme, so they are marked with (b). Your rhyme scheme for these four lines would be aabb. If you are determining the rhyme scheme for the whole poem, however, these letters would be different.
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