A circuit which generates a prescribed sequence of bits, in synchronization with a clock, is referred to as a sequence generator such generator can be used as:

1] counters.

2] Random bit generators.

3] Prescribed period and sequence generators.

4] Code generator.

The basic structure of a sequence generator is shown in diagram below:

The output Y of the next state decoder is function of $Q_N-1, Q_N-2 . . . Q_1 Q_0$ This system is similar to a ring counter or twisted ring counter which are special case of sequence generator, the design of the decoder will be clear.