diff --git a/event-schemas/schema/v1/m.room.history_visibility b/event-schemas/schema/v1/m.room.history_visibility
index bc33f4b3..9abcf5b8 100644
--- a/event-schemas/schema/v1/m.room.history_visibility
+++ b/event-schemas/schema/v1/m.room.history_visibility
@@ -1,7 +1,7 @@
 {
     "type": "object",
     "title": "Controls visibility of history.",
-    "description": "This event controls whether a member of a room can see the events that happened in a room from before they joined.",
+    "description": "This event controls whether a user can see the events that happened in a room from before they joined.",
     "allOf": [{
         "$ref": "core-event-schema/state_event.json"
     }],
@@ -12,7 +12,7 @@
                 "history_visibility": {
                     "type": "string",
                     "description": "Who can see the room history.",
-                    "enum": ["invited","joined","shared"]
+                    "enum": ["invited","joined","shared","world_readable"]
                 }
             },
             "required": ["history_visibility"]
diff --git a/specification/modules/history_visibility.rst b/specification/modules/history_visibility.rst
index f26d1b59..112e2e21 100644
--- a/specification/modules/history_visibility.rst
+++ b/specification/modules/history_visibility.rst
@@ -6,14 +6,14 @@ Room History Visibility
 This module adds support for controlling the visibility of previous events in a
 room.
 
-In all cases, a user needs to join a room to view events in that room. Once they
+In all cases except ``world_readable``, a user needs to join a room to view events in that room. Once they
 have joined a room, they will gain access to a subset of events in the room. How
 this subset is chosen is controlled by the ``m.room.history_visibility`` event
 outlined below. After a user has left a room, they may seen any events which they
 were allowed to see before they left the room, but no events received after they
 left.
 
-The three options for the ``m.room.history_visibility`` event are:
+The four options for the ``m.room.history_visibility`` event are:
 
 - ``shared`` - Previous events are always accessible to newly joined members. All
   events in the room are accessible, even those sent when the member was not a part
@@ -24,6 +24,9 @@ The three options for the ``m.room.history_visibility`` event are:
 - ``joined`` - Previous events are accessible to newly joined members from the point
   they joined the room onwards. Events stop being accessible when the member's state
   changes to something other than ``join``.
+- ``world_readable`` - All events while this is the ``m.room.history_visibility`` value
+  may be shared by any participating homeserver with anyone, regardless of whether
+  they have ever joined the room.
 
 .. WARNING::
   These options are applied at the point an event is *sent*. Checks are
@@ -43,10 +46,13 @@ Client behaviour
 Clients that implement this module MUST present to the user the possible options
 for setting history visibility when creating a room. 
 
+Clients may want to display a notice that their events may be read by non-joined
+people if the value is set to ``world_readable``.
+
 Server behaviour
 ----------------
 
-By default if no ``history_visibility`` is set the visibility is assumed to be
+By default if no ``history_visibility`` is set, or if the value is not understood, the visibility is assumed to be
 ``shared``. The rules governing whether a user is allowed to see an event depend
 solely on the state of the room *at that event*: