What defines the use of the “in-/un-” prefix for building the inversion?

Question

inexakt, but ungenau. Is this pure convention or are there any rules of thumb what prefix to choose when building a inversion of a adjective (for example pronounciation)?

Answer 1

A pretty good rule of thumb is whether the word to be negated is a Fremdwort or not. Latin words are inverted with in-, Greek words are negated with a-, while typical German words are inverted with un-.

Examples:

• Inexakt, indiskutabel, inakzeptabel, invariant, inoperabel

• ungenau, unmöglich, unwohnlich, unbehaglich, unversehrt, unverletzt, unbeschädigt, undenkbar, unfreundlich, unnahbar, unverschlossen

As usual, this is not a 100% rule. For example "Natur" is latin as well, but natürlich is negated to unnatürlich, because of the -lich suffix, which is typical for a German word.

Also note that you should avoid the un-prefix where possbile, for stylistic reasons. Almost all un-words can be expressed by an alternative word, without changing the rest of the sentence.

Some articles:

1. This first link is the best to answer your question, it also covers the a- negation and where in- turns to il-.
2. A blog posting arguing to avoid the un-words.
3. Examples for words starting with un- that are not negations or that do not have an equivalent word without the un-.

Answer 2

in- is used in Latin. Roughly: if the adjective has a Latin origin, we use in-, if not, we use un-.