Poetic lines that are uniform would be the same in key ways. The most important way for them to be uniform is rhythm/structure. For example, all lines in a sonnet will be iambic pentameter (five feet of two beats each), with each pair of syllabus sharing the same rhythms. That would produce uniform lines, and this regularity contributes to how the lines work: uniform lines create rhythm, order, structure, etc. They can also establish an expectation in readers, so that readers anticipate a specific rhythmic beat. When that shifts, the poet can use that to shock or manipulate the reader.
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