diff --git a/src/calibre/gui2/device_drivers/configwidget.ui b/src/calibre/gui2/device_drivers/configwidget.ui
index f4902a7387..619d7052e8 100644
--- a/src/calibre/gui2/device_drivers/configwidget.ui
+++ b/src/calibre/gui2/device_drivers/configwidget.ui
@@ -85,6 +85,9 @@
    </item>
    <item row="2" column="0">
     <widget class="QCheckBox" name="opt_use_subdirs">
+     <property name="toolTip">
+      <string>If checked, books are placed into sub directories based on their metadata on the device. If unchecked, books are all put into the top level directory.</string>
+     </property>
      <property name="text">
       <string>Use sub directories</string>
      </property>