diff --git a/README.md b/README.md
index 72417be..bd0e7d5 100644
--- a/README.md
+++ b/README.md
@@ -51,6 +51,8 @@ The XML is build that way:
 	[...]
 </root>
 ```
+You probably notice a custom tag:
+* **--b-- / ++b++ : It is render as BOLD in the page.
 
 ## Implement the calculator ##
 It's very easy, all you need to do is include the PHP file, the JS file and the CSS file.