I teach science to high school students. The other day I was teaching respiration to a class and the topic was Aerobic and Anaerobic Respiration. And I said Aerobic respiration is the kind of respiration that takes place in presence of oxygen. Anaerobic respiration is the kind of respiration that takes place in absence of oxygen. Here 'an' is the prefix used to denote the opposite of Aerobic. So one of my students asked me what other opposite words can be made by prefixing 'An-'. I tried googling but didn't find any such pair of opposites.

Is this the only case where An prefix makes an opposite of a word or are there others. If you could list some pairs it would be helpful.

  • Anhydrous comes to mind, Anion is another... – Solar Mike Feb 13 '19 at 17:00
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    @SolarMike: Anhydrous is, but anion isn't. That's the opposite of cation, not the opposite of ion. – SamBC Feb 13 '19 at 19:01

The 'a' prefix is really a variant of 'an' and is more common, as in:

  • moral - amoral
  • typical - atypical

"An" is used instead of "a" when the word it prefixes begins with a vowel. Neither necessarily make a straight antonym of the word it prefixes but generally create a word which is antonymous with a word sharing the same root meaning.

  • Anelectric - is something that is not electrifiable, so it is not the opposite of "electric".

  • Anacoustic - lacking in acoustic properties. Not strictly the antonym of "acoustic" because you may say that an object is an acoustic but the opposite would be to say it has acoustic properties.

  • Analgesis - a pain reliever and is the antonym of algesic, something which causes pain.

  • Anarchic - which means "without ruler", "arch" meaning ruler. We don't have the word "archic" in modern English but we do have other prefixes such as "hierarchic".

  • Analphabetic - means incapable of reading and writing, literally without alphabet. This is more antonymous with literate and is not an antonym of "alphabet".

  • Can I get some more common word examples? 'Archaic-Anarchic' is a nice one. Thank you. – YourPalNurav Feb 13 '19 at 16:33

The prefixes "an" and "a" do not create antonyms strictly speaking. They indicate a lack of something. An amoral person is a person without a canon of morals but may normally behave morally. An immoral person is a person who normally does not behave morally. Anaerobic bacteria thrive in an environment lacking oxygen, not in an environment with the opposite of oxygen (because there is no opposite to oxygen).

Furthermore, "an" and "a" are frequently applied to loanwords, which explains why English has "anarchy" but not "archy." The concept of anarchy as an abstract category of political organization was an idea that never entered the heads of the Anglo-Saxon invaders of Britain, but they had names for the thugs that led their clans and tribes. So when abstract political thought came back to England, there were existing words for kingdom, commomwealth, realm, etc. They only borrowed what they needed like "republic" and "anarchy."

  • That's exactly what's written in this table. – Lucian Sava Feb 13 '19 at 14:40

The prefix an- is equivalent to a- (as in asymmetry or asexual), the only difference being whether the word it's prefixing starts with a vowel or a consonant. Another example is anarchy, which takes the vowel-starting -archy meaning "rule" and applying the an- prefix to indicate an absence of it.

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