I just edited this answer on unix.sx. The original sentence was
But it won't transform it to an other format.
I changed this to
But it won't transform it to another format.
The second form is standard, but is the first correct?
The string an other is vanishingly rare in English. In contrast another is positively pervasive. I think it would be fair to say that the second has eclipsed the first to the point of making the first unacceptable, even though it is a grammatical string.
Both an and another are members of the category of determiners, while other, on the other hand, is an adjective. There's no grammatical reason why DET + ADJ would unacceptable. So, it must simply be a matter of convention that makes an other unacceptable.