diff --git a/src/cardos.hxx b/src/cardos.hxx
index e9cfdf1..d53936a 100644
--- a/src/cardos.hxx
+++ b/src/cardos.hxx
@@ -24,7 +24,11 @@
 #endif
 
 /** @defgroup gcardos C++ Access to Siemens CardOS 4.4
-    Implements APDUs for accessing Siemens CardOS V4.4 smartcards. */
+
+    Implements APDUs for accessing Siemens CardOS V4.4 smartcards
+    based on <a
+    href="https://de.wikipedia.org/wiki/Application_Protocol_Data_Unit">ISO
+    7816-4</a> */
 //@{
 /// @defgroup cardosexception CardOS Exceptions
 /// @defgroup cardostypes CardOS Types