One of the most significant operations of a digital system is counting. As a general rule, counting is achieved by binary numbers because of the ease with which two-state data can be controlled. Since most people are familiar with the decimal, or base 10, counting system, digital counters must make the transition from binary to decimal to be useful. Binary-coded decimal (BCD) counters are used to display decimal information in a binary code. Only the numbers 0 through 9 are displayed in binary form. Counts beyond 1001 2 or 9 10 are invalid and are not used.