The toggle, or T, flip-flop is a two-input flip-flop. The inputs are the toggle (T) input and a clock (CLK) input. If the toggle input is HIGH, the T flip-flop changes state (toggles) when the clock signal is applied. If the toggle input is LOW, the T flip-flop holds the previous state.

The standard symbol for a T flip-flop is illustrated in the figure above. The clock input may be preceded by an inverter. An inverter indicates a flip-flop will toggle on a HIGH-to-LOW transition of the clock pulse. The absence of an inverter indicates the flip-flop will toggle on a LOW-to-HIGH transition of the pulse.

We will assume an initial condition (t₀) of Q being LOW and Q being HIGH. At t₁, when the clock changes from a LOW to a HIGH, the outputs remain the same as the T input is LOW. The T input goes HIGH at t₂. At t₃, the clock changes from a LOW to a HIGH and the device changes state; Q goes HIGH and Q goes LOW. The outputs remain the same at t₄ since the device is switched only by a LOW-to-HIGH transition. At t₅, when the clock goes HIGH, Q goes LOW and Q goes HIGH; they remain that way until t₇.

Between t₃ and t₇, two complete cycles of CLK occur. During the same time period, only one cycle is observed for Q or Q. Since the output frequency is one-half the clock (input) frequency, this device can be used to divide the input frequency by 2.