diff --git a/appb.adoc b/appb.adoc
index 123f7f40..9a9a63ae 100644
--- a/appb.adoc
+++ b/appb.adoc
@@ -84,6 +84,16 @@ Each **`alias`** element contains a single element:
   </alias>
 ----
 
+In exceptional cases the **`alias`** element may contain two elements:
+
+----
+  <alias id="an_id">
+    <entry_id>Identifier of a defining entry ... </entry_id>
+    <entry_id>Identifier of another defining entry ... </entry_id>
+  </alias>
+----
+
+
 [[name-table-three-entries-ex]]
 [caption="Example B.1. "]
 .A name table containing three entries
diff --git a/history.adoc b/history.adoc
index 2e9aa32e..f138b5ac 100644
--- a/history.adoc
+++ b/history.adoc
@@ -7,6 +7,7 @@
 
 === Working version (most recent first)
 
+* {issues}509[Issue #509]: In exceptional cases allow a standard name to be aliased into two alternatives
 * {issues}501[Issue #501]: Clarify that data variables and variables containing coordinate data are highly recommended to have **`long_name`** or **`standard_name`** attributes, that **`cf_role`** is used only for discrete sampling geometries and UGRID mesh topologies, and that CF does not prohibit CF attributes from being used in ways that are not defined by CF but that in such cases their meaning is not defined by CF.
 * {issues}477[Issue #477]: Period and hyphen allowed in attribute names
 * {issues}500[Issue #500]: Appendix B: Added a **`conventions`** string to the standard name xml file format definition