Digital circuits have always been designed to operate beyond the point where they could be reliably manufactured on a consistent basis. It is a simple matter of economics: By pushing the state of the art—that is, aggressively shrinking feature sizes, then testing them and discarding those that are defective—it is possible to obtain greater numbers of ICs from a single wafer than if they are manufactured with more conservative feature sizes (cf. Section 1.8 for more discussion on this practice).