The contraction of "I am not" is grammatically unusual in that it can only go in one direction. Whereas all other persons can do either – "she is not" becoming "she's not" or "she isn't" – "I am not" in statements has to become "I'm not", never "I amn't".

But in questions the second form of contraction is the only one possible: "is she not?" (or "is not she?" as Jane Austen could say and we no longer can) can only become "isn't she?". Here "am" is parallel: "am I not?" becomes "amn't I?".

Of course it's almost never written amn't or an't, though these both exist. It's invariably written aren't these days.

In my dialect the vowel change am -> aant is the same as the change in can -> caant, so I don't feel anything odd about it. And it makes no difference whether I spell it amn't, an't, or aren't, as it's pronounced the same.

However, do Americans, or anyone with a rhotic accent, feel this is odd? Do you actually pronounce the R in aren't in this case? So does it feel like an anomalous substitution of person? Or does the alternation am ~ aren't feel normal and unexceptionable?

(This was sparked by seeing a site that claimed that ain't was historically a pronunciation of amn't – I don't know whether this is true, and at the moment don't care.)