Combinational circuits form the bulk of signal processing in digital circuits. However, they have one big drawback: they cannot perform a sequence of operations. Sequential circuits are required to introduce storage, memory, and rhythm to a circuit. But sequential circuits, and registers in particular, are important not just for their own sake. Combinational and sequential circuits are never used in isolation. Most digital circuits are pipelines of combinational circuits that lie between registers. Understanding the timing of the register is necessary to understand timing in the pipeline. And timing in the pipeline is the pinnacle of circuit level design. All digital design flows assume a deep understanding of everything that can go wrong in a pipeline.