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If you're referring to one of several lengths of fiber twisted into a single thread, yarn, or rope, you call it a ply. (So if you take 3 and twist them together into one, you'd describe the resulting line as "3-ply.") If we call a post a ply, though, I'm not sure what the verb form for creating one would be. Fiber is "spun" (as in "spinning wheel") to create a ply, and plies are combined into thread by spinning, too. So maybe we could use "spin?"