1

2

3

4

Combinational circuit

Synchronous sequential circuit.

One bit memory element

One clock delay element.

SR flip-flop

JK flip-flop

D flip-flop

T flip-flop

0

1

$$\overline A B + A\overline B $$

$$\left( {\overline {A \oplus B} } \right) \oplus \left( {\overline {A \oplus B} } \right)$$