What is a poem
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A poem is a piece of writing that partakes of the nature of both speech and song that is nearly always rhythmical, usually metaphorical, and often exhibits such formal elements as meter, rhyme, and stanzaic structure.

SHORT ANSWER:

Sum lines that rhyme.

Glad I could help you today!

i'm not sure i completely understand your question, but i will do my best.

i think the answer you are looking for is sequence. (see below)

a repeated melodic fragment would be best defined as repetition, while repeated chords (different or the same) could be a chord progression. however, you then mentioned the chord and melodic fragment beginning on higher or lower pitches, in that case, the answer could be best defined as a sequence. the definition of a sequence is as follows: "the restatement of a motif or longer melodic (or harmonic) passage at a higher or lower pitch in the same voice."

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