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

2 Answers 2


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-.


  • 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-.
  • 1
    Manchmal geht auch beides, z.B. "unakzeptabel".
    – starblue
    Commented Aug 25, 2011 at 15:49
  • 4
    'unakzeptabel' halte ich für inakzeptabel.
    – Hackworth
    Commented Aug 25, 2011 at 16:01
  • 1
    Der Duden ist da aber anderer Meinung.
    – starblue
    Commented Aug 25, 2011 at 17:51
  • 1
    german.stackexchange.com/questions/2230/… Der Duden hat viele komische Meinungen, wie's aussieht. Siehe die Kommentare unter Henrik Vogts Antwort. Abgesehen davon: Ja, man würde 'unakzeptabel' verstehen, man muss es aber nicht schön finden
    – Hackworth
    Commented Aug 25, 2011 at 21:45
  • 2
    @Hackworth: Man muss da den Online-Duden von der Online-Rechtschreibprüfung des Duden unterscheiden. Letztere ist ein Witz, z.B. wird Dissinstallation (mit zwei 2 s!) akzeptiert. Aber der Online-Duden an sich erscheint mir recht zuverlässig. (Man darf natürlich trotzdem eine gesunde Skepsis nie aufgeben!) Commented Aug 26, 2011 at 6:30

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

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