plano-, plan-, plani-

(Latin: flat, even, level, smooth)

planiform
Having a flattened shape; specifically, in anatomy, said of a joint in which the surfaces of the bones are nearly or quite plane.
planipennate
Having flat, broad wings.
planness
unexplainable (adjective), more unexplainable, most unexplainable
Impossible of being accounted for or understood: Jim's parents tried to figure out his unexplainable behavior that day in school, so they went to see his teacher to find out more.