Let $X = \prod_{j \in J} X_j$ be a D326: Cartesian product such that

(i)	$\pi_j$ is a D327: Canonical set projection on $X$ for each $j \in J$
(ii)	$E \subseteq X$ is a D78: Subset of $X$

Then \begin{equation} \pi_i(E) = \{ x_i : \{ x_j \}_{j \in J} \in E \} \end{equation}