diff --git a/docs/1b/ece108.md b/docs/1b/ece108.md
index 042515d..dcf417e 100644
--- a/docs/1b/ece108.md
+++ b/docs/1b/ece108.md
@@ -111,3 +111,11 @@ $$
 An implication can be expressed as a disjunction. As long as it is stated, it can used as its definition.
 
 $$p\implies \equiv (\neg p)\vee q$$
+
+Two **converse** propositions imply each other:
+
+$$p\implies q\text{ is the converse of }q\implies p$$ 
+
+A **contrapositive** is the negatated converse, and is **logically equivalent to the original implication**. This allows proof by contrapositive.
+
+$$\neg p\implies\neg q\text{ is the contrapositive of }q\implies p$$