oil off the gauge block using a dry pad (see the accessories section). The block is then slid perpendicularly across the other block while applying moderate pressure until they form a cruciform. Finally, the block is rotated until it is inline with the other block. After use, the blocks are re-oiled or greased to protect against corrosion. The ability of a given gauge block to wring is called ; it is officially defined as "the ability of two surfaces to adhere tightly to each other in the absence of external means." The minimum conditions for wringability are a surface finish of AA or better, and a flatness of at least . There is a formal test to measure wringability. First, the block is prepared for wringing using the standard process. The block is then slid across a reference grade ( flatness) quartz optical flat while applying moderate pressure. Then, the bottom of the gauge block is observed (through the optical flat) for oil or color. For Federal Grades 0.5, 1, and 2 and ISO grades K, 00, and 0 no oil or color should be visible under the gauge block. For Federal Grade 3 and ISO grades 1 and 2, no more than 20% of the surface area should show oil or color. This test is hard to perform on gauge blocks thinner than because they tend not to be flat in the relaxed state. Accessories The pictured accessories provide a set of holders and tools to extend the usefulness of the gauge block set. They provide a means of securely clamping large stacks together, along with reference points, scribers, and various shapes of blocks that act like caliper jaws, either external or internal. Conical-tip ones ease measurement of center-to-center distances between hole centers. A stack of gauge blocks with external caliper-jaw accessories, all clamped together, acts as a quickly assembled custom-size go or no-go gauge. A special gauge block stone that cannot damage the surface is used to remove nicks and burrs to maintain wringability. There are two wringing pads used to prepare a gauge block for wringing. The first is an oil pad, which applies a light layer of oil to the block. The second is a dry pad, which removes any excess oil from the block after the oil pad has been used. Grades Gauge blocks are available in various grades, depending on their intended use. The