Q.Draw the pin diagram of IC 7400.

A universal gate is a logic gate from which every other gate and, therefore, any digital circuit can be constructed. The NAND gate and the NOR gate are the two universal gates.

The NAND gate is an AND gate followed by an inverter, so $Y = \overline{A \cdot B}$; the NOR gate is an OR gate followed by an inverter, so $Y = \overline{A + B}$. Their power comes from the fact that inversion is built in. Using only NAND gates: a single NAND with its inputs tied together gives NOT; two NANDs give AND; three NANDs give OR; and four NANDs give an XOR. Similarly, using only NOR gates one NOR gives NOT, two give OR, three give AND, and four give an XNOR. To produce a two-input OR function you need three NAND gates, and to produce it with NOR gates you need two NOR gates.

The practical importance is large. A manufacturer can build an entire system from one type of gate, which simplifies design, reduces inventory and lowers cost. This also underlies the standard two-level realizations of Boolean expressions — a NAND is the universal gate used to implement AND–OR (SOP) logic, and a NOR is used to implement OR–AND (POS) logic.

Common ICs are the 7400 (quad two-input NAND) and the 7402 (quad two-input NOR). Understanding universal gates lets you realize any expression even when only one gate type is available.