As a general principle, machinists and toolmakers try to use a stack with the fewest number of 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). However, the stacked error is still small enough that it is often 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 more expensive.