Proof by Cases/Proof Rule/Tableau Form

Proof Rule
Let $\phi \lor \psi$ be a well-formed formula in a tableau proof whose main connective is the disjunction operator.

Let $\chi$ be a well-formed formula such that $\paren {\phi \vdash \chi}$, $\paren {\psi \vdash \chi}$.

Proof by Cases is invoked for $\phi \lor \psi$ as follows:

Also defined as
Treatments that refer to this as the rule of or-elimination tend to use the abbreviation $\lor \EE$.