Overview
{% \eta =
\begin{bmatrix}
1 & 0 & 0 & 0\\
0 & -1 & 0 & 0\\
0 & 0 & -1 & 0\\
0 & 0 & 0 & -1\\
\end{bmatrix}
%}
Which is often summarized as
{% \eta_{\mu \nu} = diag(1,-1,-1,-1) %}
{% ds^2 = \eta_{\mu \nu} dx^{\mu} dx^{\nu} %}