This is in response to Dorothea’s comment below, which deserves more than just a comment in return. She notes that I used less cake flour than all-purpose flour in the cake recipe.
The thing about flour is that its ability to absorb liquid varies wildly with the weather–especially the humidity. This is doubly annoying when you realize that baking requires precise measurements that cooking doesn’t. (This is because baking is a lot more like chemistry than cooking; Harold McGee’s books on the science of food are absolutely fascinating, if you’re interested in that kind of thing.) Anyway, I used less flour that day. After you’ve made the same recipe several times, you get a feel for what the batter or dough should look, smell, and feel like when the proportions are correct. You have to be flexible yet precise, which is difficult when you don’t use the same recipe repeatedly, and bloody near impossible the first time you try out a new recipe. For the record, this cake batter is fairly thick when it’s right. It should be pourable, but only just. You should have to spread the batter around the pan a bit.
Flour also varies wildly by region in North America. Cookwise goes into this in detail, but to make a long story short, it has to do with protein content. (Bear in mind that this applies to regionally produced flours, not as much to national brands.) Generally speaking, Canadian flours have a high protein content, whereas flours in the American South have a low protein content. This is largely determined by the kind of wheat that’s used for the flour. Durum wheat is high-protein, for example, which is why it makes good pasta. (In all the books that I’ve read, “semolina” flour is usually recommended for pasta, but I’ve seen “durum semolina,” on package labels, which can be confusing–are durum and semolina two names for the same thing or what? )
Protein affects the “toughness” of baked goods, which is why if you want to eat good homemade bread in North America, then go north; for biscuits and cakes, go south. If memory serves, Cooks Illustrated usually recommends King Arthur flour.
Just as a side note, I love Cooks Illustrated. The organization is based in New England, and as I’ve said before, these are the most anal-retentive cooks on the planet. Seriously; these people could suck the upholstery off furniture. However, when they print a recipe, it works. When they recommend a product, you can be reasonably certain it’s a good one. And the artwork on their back covers is great. (A fabulous little cafe near the lab has them framed, comprising the majority of their decor. I want to do the same for my kitchen, except I’ve got no good place to put them.)
But wait, there’s more. There’s all-purpose flour, bread flour, pastry flour, and cake flour, self-rising flour. All-purpose flour is a compromise product. It’s for people who are only going to buy one kind of flour, which is probably about 95% of the people who even bother to buy flour at all. Bread flour, pastry flour, and cake flour are fairly self-explanatory; the difference is the protein content. Self-rising flour includes leavening agents; either baking powder or baking soda. I don’t remember. Maybe it’s both. At any rate, it’s convenient, but it’s difficult to adjust a recipe if you’re using it, say, in Houston in August. (Insanely high humidity. See above.)
And that’s just a quick overview of basic white flour. I’m not even going to get started on whole-wheat flour, oat flour, barley flour, rice flour, or any of the other non-wheat types. There are plenty of good books out there.