plainer, planer, planar

The words plainer, planer, planar sound the same but have different meanings and spellings. Why do plainer, planer, planar sound the same even though they are completely different words?

The answer is simple: plainer, planer, planar are homophones of the English language.

plainer
  1. :: adjective

    comparative form of plain: more plain

planar
  1. :: adjective

    Of, relating to, or situated in a plane.

  2. :: adjective

    Flat: a planar surface.

  3. :: adjective

    Having a two-dimensional characteristic.

planer
  1. :: noun

    One that planes, especially a machine tool that is used to smooth or finish the surfaces of wood or metal.

  2. :: noun

    Printing A smooth block of wood used to level a form of type.

Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.

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About Homophones

Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.

If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").