I'll bet you're wondering what the difference is between the different flours, the answer, my friend is the amount of protein. To put it into perspective, here's the approximate amount of protein for each flour:
Bread Flour: 14 - 16%
All-Purpose (AP) Flour: 10 - 12%
Pastry Flour: 9%
Cake Flour: 7-8%
Of course, these amounts vary
For 1 cup of Pastry Flour You'll Need:
3/8 cup of All Purpose Flour
5/8 cup of Cake Flour
for 4 cups of flour
1 1/2 cups All Purpose Flour
2 1/2 cups of Cake Flour
Alas, what if you don't happen to have Cake flour? You can take 1 cup of all purpose flour, remove 1 tablespoon of the flour and add 1 tablespoon of cornstarch. This mixture will result in a slightly tougher flour.
One of our favorite recipes calling for Pastry flour is homemade puff pastry.