Additive Principle (with sets).
Given two sets \(A\) and \(B\text{,}\) if \(A \cap B = \emptyset\) (that is, if there is no element in common to both \(A\) and \(B\)), then
\begin{equation*}
\card{A \cup B} = \card{A} + \card{B}\text{.}
\end{equation*}