A rule not used in propositional logic. It goes like this:

(P>Q) * (R>S)
~Q ^ ~S
Therefore, ~P ^ ~R

"If is rains (P), then you'll get wet (Q)" and "If it's cold, then you'll shiver"
Either you're not wet, or you're not shivering.
Therefore, either it's not raining, or it's not cold.

You can turn it into a Constructive Dilemma by using Transposition. Constructive Dilemma is a perfectly fine logical rule. Hence Destructive Dilemma is valid, is simply isn't used. This is simply because it would be redundant to have both Constructive Dilemma and Destructive Dilemma.

Key:
> = If, Then
* = And
~ = Not
^ = Or
P, Q, R, S = Statements

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