canine, K9

The words canine, K9 sound the same but have different meanings and spellings. Why do canine, K9 sound the same even though they are completely different words?

The answer is simple: canine, K9 are homophones of the English language.

canine
  1. :: adjective

    Of, relating to, or characteristic of the canids.

  2. :: adjective

    Of, relating to, or being one of the pointed conical teeth located between the incisors and the first bicuspids.

  3. :: noun

    An animal of the family Canidae, especially a dog.

  4. :: noun

    One of the pointed, conical teeth located between the incisors and the first bicuspids. Also called cuspid.

K9
  1. :: noun

    A trained police dog.

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