The most common type of counter is a sequential digital logic circuit with an input line called clock and multiple output lines. Counters can be synchronous or asynchronous. Synchronous counters provide output bits that change state simultaneously, with no ripple. In asynchronous counters, the external clock pulse clocks the first flip-flop, and then each successive flip-flop is clocked by one of the outputs of the previous flip-flop. Digital counters can count either up, down, or both up and down in sequence. Depending on the type of logic, counters can be Binary, Binary coded decimal, MOD-N or Bi-quinary.