diff --git a/yaml.html.markdown b/yaml.html.markdown
index ab0f9d55..d51b2147 100644
--- a/yaml.html.markdown
+++ b/yaml.html.markdown
@@ -121,6 +121,10 @@ other_anchor: *anchor_name
 base: &base
   name: Everyone has same name
 
+# The regexp << is called Merge Key Language-Independent Type. It is is used to
+# indicate that all the keys of one or more specified maps should be inserted
+# into the current map.
+
 foo: &foo
   <<: *base
   age: 10