diff -u b/modules/field/modules/list/list.module b/modules/field/modules/list/list.module
--- b/modules/field/modules/list/list.module
+++ b/modules/field/modules/list/list.module
@@ -107,7 +107,7 @@
         '#title' => t('On label'),
         '#default_value' => $on_value,
         '#required' => FALSE,
-        '#description' => t('If left empty, no string is displayed.'),
+        '#description' => t('If left empty, "!default" will be displayed.', array('!default' => t('On'))),
         // Change #parents to make sure the element is not saved into field
         // settings.
         '#parents' => array('on'),
@@ -117,7 +117,7 @@
         '#title' => t('Off label'),
         '#default_value' => $off_value,
         '#required' => FALSE,
-        '#description' => t('If left empty, no string is displayed.'),
+        '#description' => t('If left empty, "!default" will be displayed.', array('!default' => t('Off'))),
         // Change #parents to make sure the element is not saved into field
         // settings.
         '#parents' => array('off'),