the other Saturday a little group of us headed for the nearest tram stop. We got talking. As often happens the subject got on to how long I've been in France, whether I came directly to Bordeaux and stuff. It comes up because I have an accent (American? Canadian? Belgian? Martian?) but I do pronounce things like what we do in Bordeaux.
One woman said, "Ah yes, I am from north of the Loire and it wasn't till I came here that I had any idea that in and un could conceivably be pronounced differently."
So for her there were just three nasal vowels, and in un grand pain rond, un and ain sound exactly the same. As do un and in in un bon vin blanc.
I'd read about this in the unique and unparalleled Harriette Walters books. And all of a sudden the penny dropped on something that I'd not thought about.
A friend who is very cultured, well-read and a professional story-teller did some workshops on the use of the voice. We went along to some. She insisted that en and an are pronounced differently in (subtly) different parts of the mouth. We all shrugged and tried to humour her, but Harriette Waters points out that in some parts of France there are actually 5 or 6 nasal vowels, and a clean distinction is made between en and an.