Etymology
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plani- 

word-forming element meaning "level, flat, plane," from Latin plani-, from planus "flat, level" (from PIE root *pele- (2) "flat; to spread").

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plagio- 

before vowels plagi-, word-forming element meaning "slanting, oblique," from Greek plagios "oblique, slanting," from plagos "side," from PIE *plag- "flat, spread," variant form of root *plak- (1) "to be flat."

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leio- 
scientific word-forming element meaning "smooth," from Greek leios "smooth, level, flat; plain, unembroidered; beardless." E.g. leiotrichy, in ethnology, of races, "condition of having straight, lank hair" (1924).
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