Let $(X,T)$ and $(Y, T`)$ be two topological spaces and let $A$ be a subset of $X$, and $B$ a subset of $Y$. Then

$$(A\times B)'=(A'\times \operatorname{cl}(B))\cup(\operatorname{cl}(A)\times B')$$

where $\operatorname{cl}$ is the closure and the ($'$) is the set of limit points.