diff --git a/dlib/optimization/find_max_parse_cky_abstract.h b/dlib/optimization/find_max_parse_cky_abstract.h
index 0e6825dc7..85878f2ed 100644
--- a/dlib/optimization/find_max_parse_cky_abstract.h
+++ b/dlib/optimization/find_max_parse_cky_abstract.h
@@ -24,6 +24,9 @@ namespace dlib
         T right_tag;
     };
 
+    /*!A END_OF_TREE is used to indicate that parse_tree_element::left or
+         parse_tree_element::right doesn't point to another subtree.
+    !*/
     const unsigned long END_OF_TREE = 0xFFFFFFFF;
 
     template <typename T>