Definition:Biconditional/Notational Variants

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Various symbols are encountered that denote the concept of biconditionality:

Symbol Origin
$p \iff q$
$p\ \mathsf{EQ} \ q$
$p \equiv q$ 1910: Alfred North Whitehead and Bertrand Russell: Principia Mathematica
$p = q$
$p \leftrightarrow q$
$\operatorname E p q$ Łukasiewicz's Polish notation

It is usual in mathematics to use $\iff$, as there are other uses for the other symbols.