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What's the difference between instabil and unstabil?


I always use instabil, but hear other people also use unstabil.

I assumed unstabil was wrong, but Duden has an entry for it:

Duden has also an entry for instabil:

Some Internet sources say that unstabil is used for "unstable constructions / buildings" but I feel like instabil goes perfectly well in that context, too.

Can anybody shine a light on the differences in connotation if there are any?


If you enter unstabil into Google, Google's "Did you mean" feature suggests instabil.

Also, Google Trends says that instabil is used way more often:

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Just like "unstable" and "instable", but the other way around. – Raphael Jul 3 '14 at 14:28
up vote 8 down vote accepted


This is a very common German adjective prefix used to negate the adjective's meaning. Etymology shows that un- is a prefix of very old origin which can also be found in English (unstable). Whilst grammatically correct, it is not always idiomatic to use it on any adjective (children may do so):

unrund, unbedeutend, undurchsichtig (correct) vs. unrot, unleise, unlebendig (unidiomatic but not incorrect)

There are many borderline cases where the prefix un- is used but the result may be somewhat "unschön". I would count unstabil as belonging to this category.


Amongst other foreign adjective prefixes in- like un- is used to negate the meaning. This prefix is preferably used for loanwords from Latin (or Greek). We do find its counterpart in the English language too (instability probably derived from instable).

intolerant, indiskret, indirekt

It can not be used with German adjectives that are not loanwords. Note that the spelling may change to im-, il-, ir- on adjectives starting with the letters l, m, or r (immateriell, illegal, irreal).

There is little to no difference in the meaning of a negated adjective if we had used un- or in- but using a Latin prefix on a Latin loanword is considered better style.

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I don't think instable is legal in English, actually. – Ingmar Jul 3 '14 at 11:27
@Ingmar: it's certainly not used as much as it's noun instability - but I did not want to mix this up here, and I wanted to show that the issue is not only a German related one. – Takkat Jul 3 '14 at 11:35
I agree, in principle. It's just that while Duden (grudgingly?) allows instabil, of all the English dictionaries at my disposal only the OED mentions instable, not without adding "Now rare." – Ingmar Jul 3 '14 at 12:04
Might as well chime in here as a native (US) English speaker: I wouldn't use "instable", but rather unstable exclusively. However, I think the point is best made with the noun "instability", as Takkat points out. – Milchgesicht Jul 3 '14 at 12:34
@Milchgesicht: native vote rules here :) – Takkat Jul 3 '14 at 12:44

It's pretty much a matter of personal preference. I think that, historically, unstabil was considered wrong but Duden probably added it at some point simply because it was frequently used. In a general context, at least, there is no difference in meaning; I would (and do, for what it's worth) use instabil exclusively.

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First I was like: no unstabil isn't a correct word but after having looked it up in the Duden I had to accept that it's a correct word. Weird.. – cr0 Jul 3 '14 at 10:53

They are synonyms, meaning they can be used interchangeably.

However, instabil is used more often. Here is a visualization using Google's Ngram Viewer.

enter image description here

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They are synonymous, indeed, but my Sprachgefühl wants to see tiny differences in meaning: an instabil construct once was stabil, whereas an unstabil one has always been that way. – Crissov Jul 3 '14 at 15:21

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