ないとも限らない details
write + not + you can't rule out that it will happen
kakanai to mo kagiranai
you can't rule out that it will happen; it may well happen after all
午後から雨が降らないとも限らないので、傘を持って行こう。
gogo kara ame ga furanai to mo kagiranai node, kasa o motte ikou.
You can't rule out rain this afternoon, so let's take an umbrella. neutral
同じミスをまたやらないとも限らないから、手順をメモしとこう。
onaji misu o mata yaranai to mo kagiranai kara, tejun o memo shitokou.
I might well make the same mistake again, so let me jot the steps down. casual
- past
- polite
Examples
you can't rule out that it will happen — reopens an outcome that was being treated as impossible, normally to argue for a precaution
午後から雨が降らないとも限らないので、傘を持って行こう。
gogo kara ame ga furanai to mo kagiranai node, kasa o motte ikou.
You can't rule out rain this afternoon, so let's take an umbrella. neutral
同じミスをまたやらないとも限らないから、手順をメモしとこう。
onaji misu o mata yaranai to mo kagiranai kara, tejun o memo shitokou.
I might well make the same mistake again, so let me jot the steps down. casual
大きな地震が起きないとも限りませんので、日頃の備えをお願いいたします。
ooki na jishin ga okinai to mo kagirimasen node, higoro no sonae o onegai itashimasu.
A major earthquake cannot be ruled out, so please keep your everyday preparations in order. formal
Formation
{V-nai}ともかぎらないattaches to ない-form
Notes
slot: 起きないとも限らない says it may well happen. It argues for a
precaution, so a following から／ので clause is the norm.
Related patterns
Similar
とは限らない cuts a stated generalization down ("expensive isn't always tasty"); ないとも限らない works the other way, reopening a negative expectation nobody was questioning — which is why it almost always continues into から／ので and a precaution, where とは限らない can stand as the last word of the argument.
かもしれない floats a bare possibility and is neutral about what to do with it; ないとも限らない reaches the same possibility through a double negative in order to justify acting — 雨が降るかもしれない is a forecast, 雨が降らないとも限らない is an argument for taking the umbrella.
Two ways of arguing an unwelcome possibility — かねない pins it on an agent's own act and takes a ます-stem (事故を起こしかねない), while ないとも限らない reopens an outcome that was being denied and needs the slot's own ない; only かねない can name the person to blame.
Lookalikes
Same 限る, two kana apart, opposite jobs — 謝らない限り is a conditional the sentence continues past ("unless he apologizes…"), while 謝らないとも限らない is a sentence-final judgment that he may well apologize after all; the second reverses the ない it contains, the first does not.