While "plane" itself is a complete word, it can also act as a root word with the prefix "plan-", which means "flat" or "level."
Here are some examples of words with the prefix "plan-":
* Plane (a flat surface)
* Planar (relating to a plane)
* Planetary (relating to planets, which are roughly spherical, but can be considered as planar in some contexts)
* Plankton (tiny organisms that drift in water, like a flat sheet)
* Plateau (a flat, elevated area of land)