In logic and mathematics, the **logical biconditional** (sometimes known as the **material biconditional**) is the logical connective of two statements asserting "*p* if and only if *q*", where *q* is a *hypothesis* (or *antecedent*) and *p* is a *conclusion* (or *consequent*). This is often abbreviated *p iff q*. The operator is denoted using a doubleheaded arrow (↔), a prefixed E (E*pq*), an equality sign (=), an equivalence sign (≡), or *EQV*. It is logically equivalent to (p → q) ∧ (q → p), or the XNOR (exclusive nor) boolean operator. It is equivalent to "(not p or q) and (not q or p)". It is also logically equivalent to "(p and q) or (not p and not q)", meaning "both or neither".

The only difference from material conditional is the case when the hypothesis is false but the conclusion is true. In that case, in the conditional, the result is true, yet in the biconditional the result is false.

In the conceptual interpretation, *a* = *b* means "All *a* 's are *b* 's and all *b* 's are *a* 's"; in other words, the sets *a* and *b* coincide: they are identical. This does not mean that the concepts have the same meaning. Examples: "triangle" and "trilateral", "equiangular triangle" and "equilateral triangle". The antecedent is the *subject* and the consequent is the *predicate* of a universal affirmative proposition.

In the propositional interpretation, *a* ⇔ *b* means that *a* implies *b* and *b* implies *a*; in other words, that the propositions are equivalent, that is to say, either true or false at the same time. This does not mean that they have the same meaning. Example: "The triangle ABC has two equal sides", and "The triangle ABC has two equal angles". The antecedent is the *premise* or the *cause* and the consequent is the *consequence*. When an implication is translated by a *hypothetical* (or *conditional*) judgment the antecedent is called the *hypothesis* (or the *condition*) and the consequent is called the *thesis*.

A common way of demonstrating a biconditional is to use its equivalence to the conjunction of two converse conditionals, demonstrating these separately.

When both members of the biconditional are propositions, it can be separated into two conditionals, of which one is called a *theorem* and the other its *reciprocal*. Thus whenever a theorem and its reciprocal are true we have a biconditional. A simple theorem gives rise to an implication whose antecedent is the *hypothesis* and whose consequent is the *thesis* of the theorem.

It is often said that the hypothesis is the *sufficient condition* of the thesis, and the thesis the *necessary condition* of the hypothesis; that is to say, it is sufficient that the hypothesis be true for the thesis to be true; while it is necessary that the thesis be true for the hypothesis to be true also. When a theorem and its reciprocal are true we say that its hypothesis is the necessary and sufficient condition of the thesis; that is to say, that it is at the same time both cause and consequence.

Read more about Logical Biconditional: Definition, Properties, Rules of Inference, Colloquial Usage

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“The *logical* English train a scholar as they train an engineer. Oxford is Greek factory, as Wilton mills weave carpet, and Sheffield grinds steel. They know the use of a tutor, as they know the use of a horse; and they draw the greatest amount of benefit from both. The reading men are kept by hard walking, hard riding, and measured eating and drinking, at the top of their condition, and two days before the examination, do not work but lounge, ride, or run, to be fresh on the college doomsday.”

—Ralph Waldo Emerson (1803–1882)