diff --git a/Intro_to_Solid.md b/Intro_to_Solid.md
index e668b80..a2c228c 100644
--- a/Intro_to_Solid.md
+++ b/Intro_to_Solid.md
@@ -336,3 +336,6 @@ You have to create a new mother class for these two classes like the preceding e
 
 This convention says that all classes that we use depends on the interface or abstract classes.
 Like precedents example, all methods was abstracts.
+
+>The Dependency Inversion principle states that our classes should depend upon interfaces or abstract classes instead of concrete classes and functions.
+>We want our classes to be open to extension, so we have reorganized our dependencies to depend on interfaces instead of concrete classes.
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