Plain vs. Plane: How to Choose the Right Word

Both nouns relate to flatness, but they don't mean the same thing

Passenger airplane landing at dusk
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The words "plain" and "plane" are homophones, which means they sound alike but have different meanings. "Plain" can be either a noun, adjective, or adverb, while "plane" can be a noun or a verb. Although both words can refer to flatness, one is used to describe geography while the other is used to describe geometry.

How to Use Plain

As an adjective, "plain" refers to anything that is simple, uncomplicated, common, or obvious. The noun "plain" refers to a flat, usually treeless stretch of land. Plains are one of the world's major landforms and are essential for large-scale agriculture. One of the most famous examples is the Atlantic coastal plain, which extends along the East Coast of the United States.

As an adverb, "plain" acts as an intensifier, placing emphasis on what is typically a negative quality (for example, "that was just plain dumb").

How to Use Plane

"Plane" is a noun that can refer to an airplane, a tool for smoothing wood, or a level surface. In geometry, a plane is any two-dimensional surface that extends infinitely through space. This figure can be defined by (1) three points that do not fall on a single line, (2) a line and one point that does not fall on that line, (3) two intersecting lines, or (4) two parallel lines.

As a verb, "plane" refers to the act of smoothing or creating a level surface using a plane.

In religion and esoteric teachings, "plane" sometimes refers to a state or level of conscientiousness or being. In Buddhism, for example, there are said to be 31 planes of existence, ranging all the way from the States of Deprivation to the Formless Realms of infinite space and infinite consciousness. In occult philosophy, the soul is believed to pass on to an astral plane after death. Rosicrucianism—a mystical tradition that arose in the 1600s—claims that the spirit world is divided into seven cosmic planes.

Examples

"Plain" is almost always an adjective used to modify nouns whose primary quality is that they are ordinary and undistinguished:

  • He didn't want anything fancy from the bakery, only a plain loaf of bread.
  • The girl wore a plain black dress without any frills or ornaments.

As a noun, "plain" refers specifically to an area of flat land—such as grassland, meadowland, or prairie—that does not have many trees:

  • The cows stood grazing on the plain.
  • The traveling was smooth once they reached the flat Kansas plains.

"Plane" is also a noun, but it can mean a variety of things, from a craft used for air travel to a flat surface:

  • The CEO and several others flew in a small private plane.
  • He sanded down the table until it was a completely smooth plane.

In its spiritual or religious context, "plane" typically refers to a state of being or awareness:

  • After several years of meditation, she began to feel as if her mind had reached a higher plane.

How to Remember the Difference

It's easy to confuse "plain" and "plane," especially because, as nouns, they both refer to flatness. One way to remember the difference is that "plain" is spelled with an "ai" like "train," and trains are designed to travel along smooth surfaces such as plains. Planes, on the other hand, are usually conceptual or theoretical—such as geometric planes or planes of spiritual enlightenment. In other words, a "plane" with an "a" is usually abstract.

Sources

  • Casagrande, June. "The Joy of Syntax: a Simple Guide to All the Grammar You Know You Should Know." Ten Speed Press, 2018.
  • Manser, Martin H. "Good Word Guide." A & C Black, 2007.