Situation 33.36.1. Here we are given a commutative diagram of rings

$\xymatrix{ A \ar[r] & K \\ R \ar[u] \ar[r] & B \ar[u] }$

where $K$ is a field and $A$, $B$ are subrings of $K$ with fraction field $K$. Finally, $R = A \times _ K B = A \cap B$.

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).