diff --git a/Concurrent Systems/notes/13 - CCS cose varie.md b/Concurrent Systems/notes/13 - CCS cose varie.md
index 6685529..8ba455a 100644
--- a/Concurrent Systems/notes/13 - CCS cose varie.md	
+++ b/Concurrent Systems/notes/13 - CCS cose varie.md	
@@ -41,4 +41,11 @@ But it is not yet a bisimulation.
 P –β–> P’ (challenge and reply), but we don't have (P', P') in S.
 
 So let's try with: $$S = \{ (α.P+α.P+M , α.P+M) \} ∪ Id$$
-Let's go!
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+Let's go!
+
+## Congruence
+One of the main aims of an equivalence notion between processes is to make equational reasonings of the kind: “if P and Q are equivalent, then they can be interchangeably used in any execution context”.
+
+This feature on an equivalence makes it a *congruence*
+Not all equivalences are necessarily congruences (even though most of them are
+To properly define a congruence, we first need to define an execution context, and then what it means to run a process in a context. Intuitively:
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