The words plane and plain have several meanings. Unfortunately, both have a meaning relating to flatness, and this is often the source of confusion.


The word plain has several meanings:

Simple (i.e., not elaborate).
  • a plain girl
  • a plain cake
  • a plain colour
Also simple as in apparent.
  • It is plain to see.
  • It seems quite plain to me.

Plain is an adjective in these examples. An expanse of level and low land.
  • The Russian Plain.
  • I joined the Chinese farmers as they attempted to drive the yaks across the plain in western China.
Plain is a noun in these examples.


The word plane has several meanings:

An airplane.
  • What time is your plane?

A flat surface (especially in mathematics) .
  • In a 3D space, a plane can be defined by specifying a point and a normal vector to the plane.
  • Also means a level (usually figurative).

  • I was hoping for a conversion on a higher plane.
A tool for smoothing or shaping wood (i.e., a carpenter's plane).

Also the verb to plane (i.e., to shape wood).

  • Can you plane a few inches off the top of the door?
To travel on the surface of water.
  • The car hit the puddle and planed straight into the back of the lorry.
    (also known as to aquaplane)

A Quick Test

Confusion arises mostly between plain meaning expanse of lowland and plane meaning a flat surface or a level.

A plain (lowland) is always on the ground. A plane (a surface or a level) is nearly always above the an airplane.

Therefore, if it's above the ground, it's almost certainly plane.