Plain vs. Plane: What’s the Difference?

“Plain” and “plane” are homophones; the two words sound the same although they have different meanings. “Plane,” meaning aeroplane, and “plane,” meaning a flat surface, are homonyms. These words look the same and sound the same, but they have different meanings. Get ready to memorize some definitions because there are many ways to use the words “plane” and “plain,” and both spellings have homonyms. 

Here are a few common use cases:

A higher plane | A new level of achievement

A Cartesian plane | A mathematical coordinate system named after Rene Descartes

A coastal plain | Flat land near the ocean

The astral plane | The spiritual realm

Plain and simple | Without ornament or artifice

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Etymology

Since “plane” and “plain” sound so similar, you may wonder whether they evolved from the same root word. Indeed, they did. Both words originated from a single Proto-Indo-European (PIE) root. 

According to the Online Etymology Dictionary, the adjective “plain” originated with the PIE root pele-, meaning “flat; to spread.” That evolved into planus, Latin for “flat, even, level.” In the 1100’s, planus found its way into Old French with the word plain. By 1300, English-speakers adopted the word with the definition “flat, smooth.” Later in the same century, the word took on the meaning “explicit, clear, evident.” During the late 14th century, “plain” came into favor as an adverb. As a noun, “plain” followed the same path into English. The meaning “level country, expanse of level or nearly level ground,” as in Salisbury Plain, dates from 1300. 

The word “plane” also originated with the root pele- and the Latin word planus. In some cases, the word came into English through the Old French word plain. For other meanings, such as the verb “to make smooth,” the word came from the Late Latin planare and the Old French planer, meaning “to smooth, level off; wipe away, erase.” This definition appeared in the English language around the same time as “plain” in the early 1300’s. 

The plane tree derived from a separate Proto-Indo-European root than its homonyms; it originated with plat-, meaning “to spread.” The word traveled to Western Europe by way of the Latin word platanus and Greek word platanos. From the Old French plasne, the tree acquired the name “plane” in English by the late 1300’s.

Definitions

Merriam-Webster defines the word “plain” as an adjective, noun, adverb, and verb. As a verb, plain means, “to complain,” although this usage fell out of fashion and is no longer used. 

As an adverb, plain means:

  • absolutely
  • in a unambiguous manner

As an noun, plain means:

  • an extensive area of level or rolling treeless country
  • a broad unbroken expanse
  • something free from artifice, ornament, or extraneous matter

As an adjective, plain currently means:

  • undecorated
  • pure
  • unobstructed
  • evident, obvious
  • clear
  • marked by candor
  • belonging to the masses
  • ordinary
  • characterized by simplicity
  • lacking beauty or ugliness

Plane can be used as a noun, verb, or adjective. Merriam Webster defines the adjective as, “having no elevations or depressions,” “of, relating to, or dealing with geometric planes,” or “lying in a plane.”

As a noun, plane means:

  • a plant of the plane-tree family
  • a tool for smoothing or shaping a wood surface
  • an airplane
  • one of the main supporting surfaces of an airplane
  • a surface in which if any two points are chosen a straight line joining them lies wholly in that surface
  • a flat or level surface
  • a level of existence, consciousness, or development

As a verb, plane means: 

  • to level 
  • to make smooth or even by use of a plane
  • to remove by or as if by planing
  • to work with a plane
  • to do the work of a plane
  • to fly while keeping the wings motionless 
  • to skim across the surface of the water
  • to travel by airplane

Synonyms

According to Thesaurus.com, synonyms for plain include:

  • transparent
  • honest
  • conventional
  • dull
  • ordinary
  • simple
  • traditional
  • austere
  • pure
  • simple
  • stark
  • unvarnished
  • expanse
  • field
  • grassland
  • meadow
  • plateau
  • prairie
  • steppe

Thesaurus.com lists the following as synonyms for plane:

  • even
  • flat
  • flush
  • face
  • airplane
  • craft
  • jet

Other Words and Phrases

“A plain jane” is an idiomatic expression. According to The Free Dictionary, the phrase refers to, “A female who is not considered physically attractive by societal standards.” In this phrase, the word “plain” is an adjective, meaning “ordinary” or “lacking beauty.” 

As a verb, hydroplane means “to skid on a wet surface (such as pavement) because a film of water on the surface causes the tires to lose contact with it.” According to Merriam-Webster, English-speakers in the United Kingdom also use the word “aquaplane” for this definition. 

The use of “mock plane” to describe the sycamore, Acer pseudoplatanus, dates from the late 1700’s. Lexico.com explains that the first recorded usage came from botanist Thomas Martyn. 

American anabaptists, including the Amish and Mennonites, are sometimes called “Plain people” or “Pennsylvania Plain.” 

Native American tribes located in the Great Plains region of the United States and Canada have historically been referred to as “Plains Indians” or “Indigenous peoples of the Great Plains.” In truth, these groups were far from homogenous, representing diverse traditions and six distinct American Indian language families. 

According to Dictionary.com, “in plain English” is an idiom, meaning “in clear, straightforward language.”

“As plain as the nose on your face” is another English-language idiom. Merriam-Webster defines it as “very obvious.”

An undercover police officer is often described with the adjective plainclothes, as in “a plainclothes cop.”

Practice Test

  1. You’re using tricky words. Can you speak in plane / plain English?
  2. He described the area of flat land as a plane / plain, but I think it looks like a park. 
  3. He worked with a plane / plain to smooth surfaces on the New York carpentry project.
  4. They had to complete the worksheet by plotting points on a Cartesian plane / plain. 
  5. She identified the plane/ plain tree by its bark. 
  6. They preferred a plane / plain wedding ceremony without any frills. 

Answers: 1) plain 2) plain 3) plane 4) plane 5) plane 6) plain

Sources: 

  1. www.etymonline.com/word/plain
  2. www.etymonline.com/word/plane
  3. https://www.merriam-webster.com/dictionary/plain
  4. https://www.merriam-webster.com/dictionary/plane
  5. https://idioms.thefreedictionary.com/a+plain+Jane
  6. https://news.psu.edu/story/141058/2006/11/13/research/plain-people-pennsylvania
  7. https://www.britannica.com/topic/Plains-Indian
  8. https://www.dictionary.com/browse/in-plain-english
  9. https://www.merriam-webster.com/dictionary/%28as%29%20plain%20as%20the%20nose%20on%20your%20face