---
tags:
  - Logic
  - propositional-logic
  - derivation-rules
---

The biconditional means if $P$ is the case, $Q$ is the case and if $Q$ is the case, $P$ must be the case. Thus to introduce this operator we must demonstrate both that $Q$ follows from $P$ and that $P$ follows from $Q$. We do this via two sub-proofs. 

![bi-intro.png](../img/bi-intro.png)
