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 2 blocks (a .500 block wrung to a .138 block) is preferable to a stack also totaling .638 that is composed of 4 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).