In electronics an especially synchronous digital circuits, a clock signal is a signal used to coordinate the actions of two or more circuits. A clock signal oscillates between a high and a low state, normally with a 50% duty cycle, and is usually in the form of a square wave. Circuits using the clock signal for synchronization may become active at either the rising edge, falling edge, or both edges of the clock cycle; for example, DDR SDRAM is activated by both edges.