Basic register, may be any number of bits. Typically called a flip-flop if one bit. The system clock is always running from power on to power off. 1 +-------+ +-------+ +------ clk | | | | | 0 --+ +-------+ +-------+ ^ ^ ^ ^ ^ | | | | | raising falling input | input edge edge open | closed | | new output avaiable +----------+ input | | v | register | +------------+ input ->| |-> output | | data | | data | register | | | | | | | +------------+ +----------+ v ^ output | | clk not usually shown on schematics