diff --git a/include/extinit.h b/include/extinit.h
index 2b7c05954..bbb7de345 100644
--- a/include/extinit.h
+++ b/include/extinit.h
@@ -53,8 +53,10 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 #include "extnsionst.h"
 
+/* required by: libglx */
 extern _X_EXPORT Bool noCompositeExtension;
 
+/* required by: several video drivers (eg. vmware and sis) */
 #ifdef XINERAMA
 extern _X_EXPORT Bool noPanoramiXExtension;
 #endif /* XINERAMA */