Ana - thanks! That is so interesting. I'm very confused about these sizes.
I always take an 8 at AT - but my bust is about 38 1/2. (I'm not sure how to measure my shoulders, but I have always assumed I had broad shoulders because of the way things fit. But maybe that's just because of my bust?) A 10 is definitely too big (in most other brands, too.) Yet according to the size charts at AT, their size 8 has a bust of 36. Yet it fits me. I can button up all my AT blazers and they do not look tight at all.
However - my fitted jacket from Ralph Lauren is an M (which is a 8/10) and that one is definitely fitted and snug. I can button it - but it's definitely snug. According to their size charts online, an 8 is a 36 1/2, and a 10 is a 38.
I called J. Crew and the woman said for this jacket, the size 8 has a bust of 38, and the size 10 a bust of 39.
So now I really don't know how to judge if it will fit or not. I think I'd better pass on this one since it's not returnable. In the past I've felt that J. Crew fit me a bit tight. But I really like a lot of their jackets. I need to find some time to go into the store and try some jackets on.
Thanks so much to everyone for your help!