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Possible Duplicate:
Why “unequal” but “inequality”?
Origins of negative prefixes like in-, un-, il-, ir-, dis-, a-

There are a lot of prefixes for meaning "opposite". For example, dis-, un-, in-.

  • dissatisfied
  • unstable
  • incoherent

I was wondering why there isn't just one prefix meaning "opposite" that could be used for all words. Is there a specific reason behind it? I found this link:

http://www.englishclub.com/vocabulary/prefixes.htm

It sort of explains what each prefix means, but still un- and in- seem to have a lot in common. I am just wondering why have more than one.