diff --git a/24.md b/24.md
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@@ -40,4 +40,5 @@ tags
 These tags may be present in multiple event kinds. Whenever a different meaning is not specified by some more specific NIP, they have the following meanings:
 
   - `r`: a web URL the event is referring to in some way
+  - `i`: an external id the event is referring to in some way - see [NIP-XX](XX.md)
   - `title`: title of the event
diff --git a/XX.md b/XX.md
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+++ b/XX.md
@@ -0,0 +1,31 @@
+NIP-XX
+======
+
+External Content IDs
+-------------------------
+
+`draft` `optional`
+
+There are certain established global content identifiers that would be useful to reference in nostr events so that clients can query all events assosiated with these ids.
+
+- Book [ISBNs](https://en.wikipedia.org/wiki/ISBN)
+- Podcast [GUIDs](https://podcastnamespace.org/tag/guid)
+- Movie [EIDRs](https://www.eidr.org)
+
+Since the `i` tag is already used for similar references in kind-0 metadata events it makes sense to use it for these content ids as well.
+
+
+## Supported IDs
+
+### Books:
+
+- Book ISBN: `["i", "book:isbn:123"]`
+
+### Podcasts:
+
+- Podcast Feed GUID: `["i", "podcast:guid:123"]`
+- Podcast Item GUID: `["i", "podcast:item:guid:123"]`
+
+### Movies:
+
+- Movie EIDR: `["i", "movie:eidr:123"]`
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