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The LaTeX command for iff (⟺) is \iff — if and only if (wide).
\iffWrite \iff inside math mode — that is, between $…$ for inline math or in a \[ … \] / equation block for display math. This symbol needs the amsmath package, so add \usepackage{amsmath} to your preamble.
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Use \iff in math mode after adding \usepackage{amsmath} to your preamble. For example, $A \iff B$.
It comes from the amsmath package — add \usepackage{amsmath} to your preamble.