\\ for linebreak in LaTeX math environment (not \\\\) #45

To break a line in a LaTeX environment I need to use \\\\ instead of \\.

I guess here \ plays a role of escape character. However, it is inconsistent with other places in formulas where \ does not need to be escaped.

E.g.:

\begin{align}
c_{ij} &= - \langle \psi | \sigma^x_i \sigma^x_j  | \psi \rangle \\\\
\sum_{i \neq j } c_{ij} &= 1
\end{align}

works but

\begin{align}
c_{ij} &= - \langle \psi | \sigma^x_i \sigma^x_j  | \psi \rangle \\
\sum_{i \neq j } c_{ij} &= 1
\end{align}

not (i.e. there is no line break).