Modus Tollendo Tollens

Definition
If we can conclude $$p \implies q$$, and we can also conclude $$\neg q$$, then we may infer $$\neg p$$:

$$p \implies q, \neg q \vdash \neg p$$

Its abbreviation in a tableau proof is $$\textrm{MTT}$$.

This is sometimes known as modus tollens.

Proof by Natural Deduction
By the tableau method: