diff --git a/20.md b/20.md
index 447b10a..e76942a 100644
--- a/20.md
+++ b/20.md
@@ -23,6 +23,8 @@ The `message` SHOULD start with `blocked:` if the pubkey or network address has
 
 The `message` SHOULD start with `invalid:` if the event doesn't meet some specific criteria (created_at is too far off, etc)
 
+The `message` SHOULD start with `pow:` if the event doesn't meet some proof-of-work difficulty. The client MAY consult the relay metadata at this point to retrieve the required posting difficulty.
+
 The `message` SHOULD start with `rate-limited:` if the event was rejected due to rate limiting techniques.
 
 Ephemeral events are not acknowledged with OK responses, unless there is a failure.
@@ -61,6 +63,9 @@ Event rejected, `created_at` too far off
 
     ["OK", "b1a649ebe8...", false, "invalid: event creation date is too far off from the current time. Is your system clock in sync?"]
 
+Event rejected, insufficient proof-of-work difficulty
+
+    ["OK", "b1a649ebe8...", false, "pow: difficulty 26 is less than 30"]
 
 
 Future Extensions