Why NAND and NOR are called universal gates NAND and NOR gates are often described as universal. What is the reason for this designation?

Introduction / Context:
In logic design, a universal gate is one from which any Boolean function can be built. Recognizing universality helps engineers minimize part types, simplify procurement, and design robust, flexible logic with a single gate family when needed.

Given Data / Assumptions:

• Gate types under discussion: NAND and NOR.
• Combinational logic context (no sequential storage implied).
• Positive logic conventions.

Concept / Approach:

NAND is the inversion of AND; NOR is the inversion of OR. Using De Morgan's theorems, any AND, OR, or NOT function can be synthesized by networks of only NAND gates or only NOR gates. Since AND, OR, and NOT form a functionally complete set, NAND alone or NOR alone is likewise functionally complete, which is the definition of a universal gate.

Step-by-Step Illustration:

Verification / Alternative check:

Standard textbooks and application notes provide explicit constructions of NOT, AND, OR, XOR, multiplexers, and more using only NANDs or only NORs. Practical ICs such as 7400 (quad 2-input NAND) underpin countless designs, confirming universality.

Why Other Options Are Wrong:

• Prevalence in circuits (option a) is a result, not the definition.
• Geographical ubiquity (option c) is irrelevant.
• Historical firsts (option d) do not define universality.
• No logic gate operates without power; option e is false.

Common Pitfalls:

• Assuming XOR is required for completeness. AND, OR, and NOT already form a complete set; NAND or NOR alone suffices.

Final Answer:

They can be used to construct all other types of logic gates and functions.