maureen - from Lisbon Falls, ME
The difference between cake flour and all-purpose flour has to do with the amount of protein in the flour. Cake flour generally has about 6-8% protein and all-purpose has about 11%. Lower protein means a more tender cake, which is why some recipes call for cake flour instead of all-purpose flour.
In most recipes, it is okay to substitute all-purpose flour, but the ratio is not 1:1. For 1 cup of cake flour, subsitute 1 cup MINUS 2 tablespoon of all-purpose flour. Because of the higher protein content, you need to use a little bit less flour.
This substitution will work in nearly all recipes, but your cake will not be quite as tender as it would have been if made using cake flour.