A flip-flop is a circuit that stores a single bit. Its defining feature is that it has two stable states and will hold whichever one it is in until something deliberately changes it. This is achieved by cross-coupling two amplifying elements, originally vacuum tubes, later logic gates, so that the output of each feeds the input of the other. The feedback locks the pair into one of two self-reinforcing conditions, which we read as a stored 0 or a stored 1. A simpler relative without a clock input is called a latch; a flip-flop proper changes state only in response to a clock signal.
The circuit dates to 1918, when the British engineers William Eccles and Frank Jordan filed the patent later published as GB148582, “Improvements in Ionic Relays.” Their patent describes a pair of thermionic valves connected so that, as the specification puts it, the amplified potential change in the anode circuit of the last valve is communicated back to the control electrode of the first valve. That regenerative feedback gives the circuit its two stable conditions. The arrangement became known as the Eccles-Jordan trigger circuit and is the direct ancestor of every flip-flop since.
The importance of the flip-flop is that it gives a digital circuit memory. Pure combinational logic, networks of logic gates as analyzed in Boolean terms, has no way to remember anything: its outputs depend only on its present inputs. By adding flip-flops, designers create sequential logic, in which the circuit’s behavior depends on its history as well as its current inputs. The stored state, advanced one step at a time by the clock, is what lets a machine carry information forward from one moment to the next.
Flip-flops are the cells from which larger storage is assembled. A group of flip-flops sharing a clock forms a register, the small fast storage inside a processor that holds operands and addresses. Counters, shift registers, and the state machines that sequence a processor’s control are all built from flip-flops. Every time a clock edge arrives, a bank of flip-flops captures the freshly computed values from the surrounding combinational logic and presents them as the starting point for the next cycle.
What began as a way to make a more sensitive relay turned out to be the atom of digital memory. From two cross-coupled valves drawn in a 1918 patent to the billions of storage cells on a modern chip, the principle is unchanged: feedback creates two stable states, and two stable states store one bit.