diff --git a/README.md b/README.md
index c0779f4..bfc3927 100644
--- a/README.md
+++ b/README.md
@@ -392,6 +392,13 @@ Indicates schema general information.
 
 Mandatory.
 
+## References
+
+* "XSD Tutorial", [Tutorials Point].
+* "XML Schema Tutorial", [W3Schools].
+
   [YASD]: https://gitlab.com/amlengua/apal/esquema/-/blob/main/apal.yaml
   [XSD]: https://gitlab.com/amlengua/apal/esquema/-/blob/main/apal.xsd
   [Markdown]: https://gitlab.com/amlengua/apal/esquema/-/blob/main/apal.md
+  [Tutorials Point]: https://www.tutorialspoint.com/xsd/
+  [W3Schools]: https://www.w3schools.com/xml/schema_intro.asp