\begin{definition}[Definition:Set Union/Family of Sets/Two Sets]
Let $I = \set {\alpha, \beta}$ be an indexing set containing exactly two elements.
Let $\family {S_i}_{i \mathop \in I}$ be a family of sets indexed by $I$.
From the definition of the union of $S_i$:
:$\ds \bigcup_{i \mathop \in I} S_i := \set {x: \exists i \in I: x \in S_i}$
it follows that:
:$\ds \bigcup \set {S_\alpha, S_\beta} := S_\alpha \cup S_\beta$
\end{definition}