What Are Disjoint Events in Probability?

You roll a die. Let $A$ be the compound event defined as “more than four dots”, $A=\left\{5,6\right\}$, and $B$ be “fewer than three dots”, $B=\left\{1,2\right\}$. That means the intersection between $A$ and $B$ is the empty set, because they have no outcomes in common. They can never both occur at once.

You denote the empty set with the symbol $\varnothing$.

Note! Complementary outcomes are always disjoint, because an event can not both happen and not happen at the same time.