diff --git a/config.def.h b/config.def.h
index 77ff358..8fd5d4a 100644
--- a/config.def.h
+++ b/config.def.h
@@ -17,6 +17,10 @@ static const Bool topbar            = True;     /* False means bottom bar */
 static const char *tags[] = { "1", "2", "3", "4", "5", "6", "7", "8", "9" };
 
 static const Rule rules[] = {
+	/* xprop(1):
+	 *	WM_CLASS(STRING) = instance, class
+	 *	WM_NAME(STRING) = title
+	 */
 	/* class      instance    title       tags mask     isfloating   monitor */
 	{ "Gimp",     NULL,       NULL,       0,            True,        -1 },
 	{ "Firefox",  NULL,       NULL,       1 << 8,       False,       -1 },