How to write if and only if symbol / equivalence in Latex ? In mathematics, if and only if (iff) is a biconditional logical connective between statements, where either both statements are true or both are false.

How to write if and only if symbol / equivalence in Latex ?

You do not have to use any package:

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\begin{document}
\noindent
A $\Leftrightarrow$ B \\
C $\Longleftrightarrow$ D
\end{document}

For a short if and only if, use \Leftrightarrow: A $\Leftrightarrow$ B

For a long if and only if, use \Longleftrightarrow: C $\Longleftrightarrow$ D

Liste of all arrows

If you want to see all type of Latex arrows, have a look to Latex Arrows.