be kept at this temperature when taking measurements. This mitigates the effects of thermal expansion. The wear blocks, made of a harder substance like tungsten carbide, are included at each end of the stack, whenever possible, to protect the gauge blocks from being damaged in use. Machinists and toolmakers try to use a stack with the fewest blocks to avoid accumulation of size errors. For example, a stack totaling .638 that is composed of two blocks (a .500 block wrung to a .138 block) is preferable to a stack also totaling .638 that is composed of four blocks (such as a .200, .149, .151, and .138 all wrung together). As detailed in the Grades section, each block has a size tolerance of a few millionths of an inch, so stacking them together introduces a cumulative uncertainty. However, the stacked error from even multiple blocks is usually negligible in all but the most demanding uses. In a busy shop, some of the blocks will be in use elsewhere, so one creates a stack from the blocks available at the time. Typically the few millionths of an inch difference will not be detectable, or matter, in the context. Contexts demanding ultimate precision are rarer and require additional expense (for example, more sets of blocks and higher grades of blocks). Wringing Wringing is the process of sliding two blocks together so that their faces bond. Because of their ultraflat surfaces, when wrung, gauge blocks adhere to each other tightly. Properly wrung blocks may withstand a pull. The mechanism is a combination of: Vacuum applies pressure between the blocks because the air is squeezed out of the joint Surface tension from oil and water vapor that is present between the blocks Molecular attraction that occurs when two very flat surfaces are brought into contact; this force causes gauge blocks to adhere even without surface lubricants, and in a vacuum It is believed that the last two sources are the most significant. There is no magnetism involved, although to a user the clinging together of the blocks feels a bit like weak refrigerator magnets sticking together. Unlike magnets, however, the cling only lasts while the blocks are completely joined—the blocks do not attract each other across any visible gap, as magnets would. The process of wringing involves four steps: Wiping a clean gauge block across an oiled pad (see the accessories section). Wiping any extra