In- vs. Un- Which Prefix should I use?

in- vs. un-
image by brett jordan

The English language can be very tricky, especially for people that are just learning the language. But even native speakers have trouble with certain aspects, rules, or irregularities. There are many rhymes and rules for grammar and spelling in English – just as there are in any other language –  yet, there are just as many exceptions to those rules. This is where learning can become frustrating.


One quite difficult aspect is how to choose the correct prefix for a word to change its meaning. In this article, we want to look at the cases in which “in-” and “un-” are used to give a word another (negative) meaning.


In- vs. Un-


Before we look at the rules and strategies to find out which to use, “in-” or “un-“, let’s have a look at what they are used for. “In-” as well as “un-” are s-called prefixes. A prefix consists of letters that are added to the beginning of a word (hence the “pre”). They carry meaning and can change the meaning of the so-called root word they are added to. Prefixes like “in-” and “un-” are added to words to express the negation of said word, aka the not-state of it.

For example “impossible” express the state of something being “not possible”.


The Rule

Of course there is a rule in the English language about when to use “in-” or “un-“, yet this rule is not only hard for language learners. To understand which prefix to use, it’s necessary to know the language the word in question comes from – and this is a hard task for natives and non-natives alike! It’s difficult to determine the root word’s origin, and just as hard to try and remember them all.


Basically, words of English or rather Germanic (no, not German, but the ancient language stem both English and German originate from) origin take the prefix “un-“, while words that came into the English language from Latin take “in-“. The same goes for the forms “im-“, “il-“, and “ir-“, by the way. They are also added to Latin stemmed words since they are all variations from “in-“, their choice only determined by the beginning of the word they are added to.

“Il-” is used when the root word starts with “l”, e.g. illegal.

“Im-” is used when added to words beginning with “m”, “b”, or “p”, e.g. immature, impossible.

And “ir-” is used when the root word starts with an “r” as well, e.g. irregular.


So, either you are very skilled when it comes to language and have no difficulty in finding out whether a word is of English or Latin origin, or you really have to learn these words by hard to add the correct prefix.


Problematic cases

Throughout the history of English, there had been a vigorous battle going on – despite this rule! For centuries, words in both versions, like inability and unability have been co-existing (the first one being the nowadays correct version). Most of these cases are cleared by now, with definite lexicon entries that tell you which form to use.


Yet, there are still a few pairs that haven’t been assigned an ultimate correct version, for example inarguable and unarguable. It becomes even more problematic when different prefixes are used for the noun and corresponding adjectives, a famous example being uncivil, the adjective formed using the “un-” prefix, and incivility, the thus formed noun taking “in-” instead.


For these cases, there is no easy way to remember or make up a rule. They simply have to be remembered by the learner. However, since Latin is not spoken anymore and words with Latin roots have already been introduced to the language (including the “in-” prefix), the prefix itself is not very lively anymore. Today, the negation using the prefix “un-” is far more common.


Other Prefixes

It would be easy if we would be done with that, right? But we aren’t. Despite “un-” and “in-” with it’s different forms, there are other prefixes in English that express negotiation.

“A-“, “an-“, and their extended variations “ana-” and “anti-” can be dealt with like “in-” with the difference that they are added to words with a Greek origin rather than Latin.

Prefixes like “de-“, “dys-“, and “dis-” are of Latin origin as well but their meaning differs from the “pure” negation as it has been the case in what we have discussed before.



Usually, prefixes are added directly to the root word, yet, in some cases, the use of a hyphen is necessary. This occurs when the prefix is added to an already proper noun, for example un-American. Furthermore, a hyphen is used to separate two vowels, e.g. anti-intellectual.