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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`: name of [NIP-51](51.md) sets, [NIP-52](52.md) calendar event, [NIP-53](53.md) live event or [NIP-99](99.md) listing
diff --git a/XX.md b/XX.md
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+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 [ISANs](https://en.wikipedia.org/wiki/International_Standard_Audiovisual_Number)
+
+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", "isbn:9780765382030"]` - https://isbnsearch.org/isbn/9780765382030
+
+Book ISBNs MUST be referenced _**without hyphens**_ as many book search APIs return the ISBNs without hyphens. Removing hypens from ISBNs is trivial, whereas adding the hyphens back in is non-trivial requiring a library.
+
+### Podcasts:
+
+- Podcast RSS Feed GUID: `["i", "podcast:guid:c90e609a-df1e-596a-bd5e-57bcc8aad6cc"]` - https://podcastindex.org/podcast/c90e609a-df1e-596a-bd5e-57bcc8aad6cc
+- Podcast RSS Item GUID: `["i", "podcast:item:guid:d98d189b-dc7b-45b1-8720-d4b98690f31f"]`
+
+### Movies:
+
+- Movie ISAN: `["i", "isan:0000-0000-401A-0000-7"]` - https://web.isan.org/public/en/isan/0000-0000-401A-0000-7
+
+Movie ISANs SHOULD be referenced  _**without the version part**_ as the versions / edits of movies are not relevant. More info on ISAN parts here -  https://support.isan.org/hc/en-us/articles/360002783131-Records-relations-and-hierarchies-in-the-ISAN-Registry
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