What does "nib" and "xib" stand for?

Question

I wonder why that file extension is called "nib" or "xib".

I guess that the "ib" in "nib" is "Interface Builder". But the n? Or the x? Any idea?

Answer 1

I think the "n" would mean NextStep, the "x" was added to indicate the new XML based format.

Answer 2

nib stands for "NeXT Interface Builder", whereas xib is for "Mac OS X Interface Builder".

Answer 3

What do they do?