diff --git a/10.md b/10.md
index c0ef9d19..64947967 100644
--- a/10.md
+++ b/10.md
@@ -43,8 +43,8 @@ They are citings from this event.  `root-id` and `reply-id` are as above.
 Where:
 
  * `<event-id>` is the id of the event being referenced.
- * `<relay-url>` is the URL of a recommended relay associated with the reference.  It is NOT optional.
- * `<marker>` is optional and if present is one of `"reply"`, `"root"`, or `"mention"`
+ * `<relay-url>` is the URL of a recommended relay associated with the reference. Clients SHOULD add a valid `<relay-URL>` field, but may instead leave it as `""`.
+ * `<marker>` is optional and if present is one of `"reply"`, `"root"`, or `"mention"`.
 
 **The order of marked "e" tags is not relevant.**  Those marked with `"reply"` denote the id of the reply event being responded to.  Those marked with `"root"` denote the root id of the reply thread being responded to. For top level replies (those replying directly to the root event), only the `"root"` marker should be used. Those marked with `"mention"` denote a quoted or reposted event id.