A shift register is a type of digital circuit using a cascade of flip flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one location to the next. By connecting the last flip-flop back to the first, the data can cycle within the shifters for extended periods, and in this form they were used as a form of computer memory. In this role they are very similar to the earlier delay line memory systems and were widely used in the late 1960s and early 1970s to replace that form of memory.