diff --git a/1/LMSGI/biblioteca/biblioteca.dtd b/1/LMSGI/biblioteca/biblioteca.dtd
new file mode 100644
index 0000000..34d7b63
--- /dev/null
+++ b/1/LMSGI/biblioteca/biblioteca.dtd
@@ -0,0 +1,6 @@
+<!ELEMENT library (book+)>
+<!ELEMENT book (title,author,lang,year)>
+<!ELEMENT title (#PCDATA)>
+<!ELEMENT author (#PCDATA)>
+<!ELEMENT lang (#PCDATA)>
+<!ELEMENT year (#PCDATA)>
diff --git a/1/LMSGI/biblioteca/biblioteca.xml b/1/LMSGI/biblioteca/biblioteca.xml
index 39d24f7..aaed6f8 100644
--- a/1/LMSGI/biblioteca/biblioteca.xml
+++ b/1/LMSGI/biblioteca/biblioteca.xml
@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <?xml-stylesheet href="style.css" type="text/css"?>
+<!DOCTYPE library SYSTEM "biblioteca.dtd">
 <library>
 	<book>
 		<title>Ishmael</title>