Left Right Double Arrow ⇔

The Left Right Double Arrow character (⇔) is a standard HTML entity defined in the HTML specification. In HTML source code, it can be written using the named entity reference ⇔, the decimal numeric character reference ⇔, or the hexadecimal numeric reference ⇔. The character is assigned Unicode code point U+21D4 in the Universal Character Set.

The left-right double arrow is the standard symbol for logical biconditional (if and only if) in mathematical logic. It features double arrow lines pointing in both directions. In formal mathematics, it indicates logical equivalence — that two statements have the same truth value under all conditions.

Arrow entities serve as directional indicators in navigation interfaces, mathematical expressions, flowcharts, and textual content throughout the web. Because they render as scalable text characters rather than bitmap images, HTML arrow entities are resolution-independent, styleable with CSS properties like color and font-size, and fully accessible to assistive technologies including screen readers.

When deciding how to encode the Left Right Double Arrow character in your HTML documents, the named entity ⇔ is generally the most readable choice for developers reviewing or maintaining source code. The decimal form ⇔ and hexadecimal form ⇔ are equally valid alternatives that work in contexts where named entities may not be supported, or when generating HTML output programmatically from server-side code. All three representations produce identical visual output in every modern web browser.

Use the left-right double arrow for logical biconditional (A ⇔ B means 'A if and only if B'), logical equivalence in proofs, and formal specifications. It is stronger than the single bidirectional arrow and is the standard notation for 'iff' in mathematics.