Remark 104.4.2. We have the formula $S \times _{\mathcal{M}_{1, 1}} S' = (S \times S') \times _{\mathcal{M}_{1, 1} \times \mathcal{M}_{1, 1}} \mathcal{M}_{1, 1}$. Hence the key fact is a property of the diagonal $\Delta _{\mathcal{M}_{1, 1}}$ of $\mathcal{M}_{1, 1}$.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).