Flour labeled 00, also known as doppio zero flour, is a very finely ground flour that is often used for making pastries, pastas and breads. In the United States, it is most commonly ground from durum wheat.
The protein content of 00 flour is usually similar to that of all-purpose white flour, but the gluten in it acts differently. This makes foods made from 00 flour easier to chew.
Generally, 00 and all-purpose flour can be substituted for one another. The result will be a finer texture when 00 is used instead of all-purpose flour, and a less fine texture when all-purpose flour is substituted for 00 flour.