In real systems, don’t cares occur in several ways. First, there may be some input combinations that never occur. That is the case in CE4,where the input is the code for a decimal digit; there are only 10 possible input combinations. If a 4-bit code is used, then six of the input combinations never occur. When we build a system, we can design it such that the outputs would be either 0 or 1 for each of these don’t care combinations, since that input never happens.