Q. What do you mean by Sequential Blocks?

Neglecting propagation delays, which are measures of how long it takes the output of a gate to respond to a transition at the input of the gate, the output of a logic block at a given time depends only on the inputs at that same time. The output of a sequential block, on the otherhand, depends not only on the present inputs but also on inputs at earlier times. Sequential blocks have thus a kind of memory, and some of them are used as computer memories.

Most sequential blocks are of the kind known as multivibrators, which can be monostable (the switch remains in only one of its two positions), bistable (the switch will remain stable in either of its two positions), and unstable (the switch changes its position continuously as a kind of oscillator, being unstable in both of its two states). The most common sequential block is the flip-flop, which is a bistable circuit that remembers a single binary digit according to instructions.

Flip-flops are the basic sequential building blocks. Various types of flip-flops exist, such as the SR flip-flop (SRFF), D flip-flop (or latch), and JK flip-flop (JKFF), which differ from one another in the way instructions for storing information are applied.