A. The #20 or #12 or similar indicates the U.S. Mesh Size (or U.S. Sieve Size) that tells you how fine the mesh is - in other words, how many holes there are per square inch of the screen. For instance, a #20 mesh will have 20 holes in one square inch of the screen. The larger the number, the smaller the holes will be or the finer the mesh will be. To provide some point of reference, consider that a #12 mesh has about a 1700 micron rating, which is about the same diameter as an average strand of spaghetti. A #20 mesh has a micron rating of about 850 - or close to the size of a grain of sand.