diff --git a/youtube_dl/extractor/common.py b/youtube_dl/extractor/common.py
index 77726ee24..b577c9e6d 100644
--- a/youtube_dl/extractor/common.py
+++ b/youtube_dl/extractor/common.py
@@ -52,8 +52,19 @@ class InfoExtractor(object):
     view_count:     How many users have watched the video on the platform.
     urlhandle:      [internal] The urlHandle to be used to download the file,
                     like returned by urllib.request.urlopen
-
-    The fields should all be Unicode strings.
+    formats:        A list of dictionaries for each format available, it must
+                    be ordered from worst to best quality. Potential fields:
+                    * url       Mandatory. The URL of the video file
+                    * ext       Will be calculated from url if missing
+                    * format    A human-readable description of the format
+                                ("mp4 container with h264/opus").
+                                Calculated from width and height if missing.
+                    * format_id A short description of the format
+                                ("mp4_h264_opus" or "19")
+                    * width     Width of the video, if known
+                    * height    Height of the video, if known
+
+    Unless mentioned otherwise, the fields should be Unicode strings.
 
     Subclasses of this one should re-define the _real_initialize() and
     _real_extract() methods and define a _VALID_URL regexp.