Why do we say undeletable instead of indeletable. Sometimes with other words we use an in prefix, such as with indefatigable.

I'm curious about un vs in.

  • You make it sound like we don't use "un-" ever... "unmentionable" means "unable to be mentioned"... why do you think "in" is better? Also... we do actually use a form of "in-" with delete - indelible
    – Catija
    Commented Jun 16, 2016 at 21:22
  • I'm not entirely sure. I'll edit the question to remove any suggestion that 'un' isn't ever used
    – danwellman
    Commented Jun 16, 2016 at 21:26
  • 3
    I suggest caution in using "undeletable" because, thanks to computers, there is a verb "undelete". As a result, "undeletable" seems to have not only its traditional meaning, "unable to be deleted", but also the meaning "able to be undeleted". Commented Jun 16, 2016 at 23:22
  • 5
    The proper word is indelible. The word undeletable is computerese, and should only be used in the context of computers. And, as @Andreas comments, it's ambiguous in that context. Commented Jun 17, 2016 at 2:49
  • 1
    Possible duplicate of Why "unequal" but "inequality"?
    – MetaEd
    Commented Jun 27, 2016 at 19:13

1 Answer 1


An article on Mental Floss discusses and explains this: http://mentalfloss.com/article/57840/whats-difference-between-and-un

To sum it up, there is no fixed rule between using "un-" and "in-". The former usually works on words with Germanic roots, and the latter works on words with Latin roots. Making this distinction is not mandatory, but it seems to be the case.

In most cases, one prefix will be more suitable than the other. This is determined by what either prefix adds to the word: connotation, pronunciation, e.t.c. "Infinite" is a common word whereas "unfinite" is never used, though the prefixes have the same meaning. "Intouchable" is never used, whereas "untouchable" is.

Again, there is only a fine line between the two. Unfortunately, it can be extremely bothersome to differentiate. My advice is to use whichever conveys your thoughts better, and does not sound odd.

  • ... therefore, we ought to say infortunately, not unfortunately :-) (since 'fortuna' is clearly good old Latin) Oh well, just having fun at your expense. As a non-native English speaker, I ever so often hesitate if a word is used with un- or in- and I always wondered what was the difference or even why there was one. Now that you saw your answer, I remembered that in German un- is frequent with non-Latin words, as in, say, Unmenschlich (inhuman), so I guess that your answer is correct for English as well. Commented Jul 18, 2020 at 16:13
  • Yeah, with English, unfortunately, you sorta have to learn grammatical rules for words coming from multiple language families, and sometimes the rules aren't always used consistently Commented Nov 1, 2021 at 15:31

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