0x21D4

0x21D4 is the hexadecimal notation for the Unicode code point U+21D4, which encodes the symbol ⇔, known as the left–right double arrow. This character appears in the Mathematical Operators block of Unicode and is used primarily in mathematics and logic to denote a biconditional or logical equivalence.

In formal notation, the symbol ⇔ expresses that two statements or propositions are mutually entailing each other: P ⇔ Q is read as “P if and only if Q.” It is commonly used to indicate that a relation is true in both directions or to denote definitional equivalence, where two expressions define the same concept.

Encoding and rendering details: U+21D4 resides in the BMP (Basic Multilingual Plane) and is represented in UTF-8 as the three-byte sequence E2 9D 94. In HTML, it can be referenced as ⇔ or ⇔. Rendering of the symbol depends on the font and rendering engine; some fonts display a double-lined left-right arrow, while others may render with slight stylistic differences.

Related symbols include the single left-right arrow U+2194 (↔), which differs in its line style, and other biconditional or equivalence indicators used in logic and computer science.