diff --git a/beets/library.py b/beets/library.py
index 4b4563d60..7d02c32d1 100644
--- a/beets/library.py
+++ b/beets/library.py
@@ -333,11 +333,12 @@ class Item(LibModel):
     _flex_table = 'item_attributes'
     _search_fields = ITEM_DEFAULT_FIELDS
 
-    media_fields = set(MediaFile.readable_fields()).intersection(ITEM_KEYS)
-    """Set of property names to read from ``MediaFile``.
+    _media_fields = set(MediaFile.readable_fields()).intersection(ITEM_KEYS)
+    """Set of item fields that are backed by `MediaFile` fields.
 
-    ``item.read()`` will read all properties in this set from
-    ``MediaFile`` and set them on the item.
+    Any kind of field (fixed, flexible, and computed) may be a media
+    field. Only these fields are read from disk in `read` and written in
+    `write`.
     """
 
     @classmethod
@@ -393,7 +394,7 @@ class Item(LibModel):
         """Read the metadata from the associated file.
 
         If ``read_path`` is specified, read metadata from that file
-        instead. Updates all the properties in ``Item.media_fields``
+        instead. Updates all the properties in `_media_fields`
         from the media file.
 
         Raises a `ReadError` if the file could not be read.
@@ -407,7 +408,7 @@ class Item(LibModel):
         except (OSError, IOError) as exc:
             raise ReadError(read_path, exc)
 
-        for key in list(self.media_fields):
+        for key in list(self._media_fields):
             value = getattr(mediafile, key)
             if isinstance(value, (int, long)):
                 # Filter values wider than 64 bits (in signed representation).
diff --git a/beets/plugins.py b/beets/plugins.py
index 18bc3290c..4b4a0d090 100755
--- a/beets/plugins.py
+++ b/beets/plugins.py
@@ -111,7 +111,7 @@ class BeetsPlugin(object):
         # Defer impor to prevent circular dependency
         from beets import library
         mediafile.MediaFile.add_field(name, descriptor)
-        library.Item.media_fields.add(name)
+        library.Item._media_fields.add(name)
 
     listeners = None
 
diff --git a/beets/ui/commands.py b/beets/ui/commands.py
index f22837d05..d83f60a77 100644
--- a/beets/ui/commands.py
+++ b/beets/ui/commands.py
@@ -919,7 +919,7 @@ def update_items(lib, query, album, move, pretend):
 
             # Check for and display changes.
             changed = ui.show_model_changes(item,
-                                            fields=library.Item.media_fields)
+                                            fields=library.Item._media_fields)
 
             # Save changes.
             if not pretend:
@@ -1246,7 +1246,7 @@ def write_items(lib, query, pretend):
 
         # Check for and display changes.
         changed = ui.show_model_changes(item, clean_item,
-                                        library.Item.media_fields,
+                                        library.Item._media_fields,
                                         always=True)
         if changed and not pretend:
             try:
diff --git a/test/test_mediafile.py b/test/test_mediafile.py
index bcf80ccbe..6f86f6c88 100644
--- a/test/test_mediafile.py
+++ b/test/test_mediafile.py
@@ -274,7 +274,7 @@ class ExtendedFieldTestMixin(object):
         mediafile = MediaFile(mediafile.path)
         self.assertEqual(mediafile.initialkey, 'F#')
         delattr(MediaFile, 'initialkey')
-        Item.media_fields.remove('initialkey')
+        Item._media_fields.remove('initialkey')
 
     def test_write_extended_tag_from_item(self):
         plugin = BeetsPlugin()
@@ -289,7 +289,7 @@ class ExtendedFieldTestMixin(object):
         self.assertEqual(mediafile.initialkey, 'Gb')
 
         delattr(MediaFile, 'initialkey')
-        Item.media_fields.remove('initialkey')
+        Item._media_fields.remove('initialkey')
 
     def test_read_flexible_attribute_from_file(self):
         plugin = BeetsPlugin()
@@ -302,7 +302,7 @@ class ExtendedFieldTestMixin(object):
         self.assertEqual(item['initialkey'], 'F#')
 
         delattr(MediaFile, 'initialkey')
-        Item.media_fields.remove('initialkey')
+        Item._media_fields.remove('initialkey')
 
     def test_invalid_descriptor(self):
         with self.assertRaises(ValueError) as cm: