As for haiku, originally it required 575, where each number indicates the number of syllables (not words) in the corresponding line. Later they relaxed the requirement (in terms of syllable count). Right looks like there is a grammatical change or something, but I never delved into it. Classic poems are much more difficult to write.
Of course there are many nonclassic poems that are just as good such as the raven (from Poe). Whitman might be the first who broke the ground in free verse. Certainly there are music and rhythm in those.
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