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The LaTeX command for Leftrightarrow (⇔) is \Leftrightarrow (if and only if).
\LeftrightarrowWrite \Leftrightarrow inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
$A \Leftrightarrow B$Prefer to see it live? Paste it into the LaTeX preview and it renders as you type: no compiler, no signup.
\to\leftarrow\rightarrow\leftrightarrow\Leftarrow\Rightarrow\iff\implies\uparrow\downarrow\updownarrow\nearrow\searrow\swarrowSearch all 380+ LaTeX symbols →
Use \Leftrightarrow in math mode; it needs no extra package. For example, $A \Leftrightarrow B$.
None: \Leftrightarrow is built into LaTeX, so no extra package is required.