Combinational and sequential circuits

In June 1918 the British physicists William Eccles and F. W. Jordan applied for a patent on a circuit of two vacuum tubes wired so that each held the other in its state. Pushed one way, it stayed there until pushed back. They described it as a trigger relay for radio work; the circuit became known as the flip-flop.

A circuit that remembers one bit is the difference between a calculator and a computer. Every register, counter and cache in a processor is built from that idea, connected to logic that has no memory at all.