diff --git a/24.md b/24.md
index 3adec243..ef4196bd 100644
--- a/24.md
+++ b/24.md
@@ -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-73](73.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/73.md b/73.md
new file mode 100644
index 00000000..12228d36
--- /dev/null
+++ b/73.md
@@ -0,0 +1,48 @@
+NIP-73
+======
+
+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"]`
+- Podcast RSS Publisher GUID: `["i", "podcast:publisher:guid:18bcbf10-6701-4ffb-b255-bc057390d738"]`
+
+### 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
+
+---
+
+### Optional URL Hints
+
+Each `i` tag MAY have a url hint as the second argument to redirect people to a website if the client isn't opinionated about how to interpret the id:
+
+`["i", "podcast:item:guid:d98d189b-dc7b-45b1-8720-d4b98690f31f", https://fountain.fm/episode/z1y9TMQRuqXl2awyrQxg]`
+
+`["i", "isan:0000-0000-401A-0000-7", https://www.imdb.com/title/tt0120737]`
+
+
diff --git a/README.md b/README.md
index 550888a8..6c83ba88 100644
--- a/README.md
+++ b/README.md
@@ -77,6 +77,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 - [NIP-70: Protected Events](70.md)
 - [NIP-71: Video Events](71.md)
 - [NIP-72: Moderated Communities](72.md)
+- [NIP-73: External Content IDs](73.md)
 - [NIP-75: Zap Goals](75.md)
 - [NIP-78: Application-specific data](78.md)
 - [NIP-84: Highlights](84.md)
@@ -238,7 +239,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 | `-`               | --                                   | --                              | [70](70.md)                           |
 | `g`               | geohash                              | --                              | [52](52.md)                           |
 | `h`               | group id                             | --                              | [29](29.md)                           |
-| `i`               | identity                             | proof                           | [39](39.md)                           |
+| `i`               | external identity                    | proof, url hint                 | [39](39.md), [73](73.md)              |
 | `k`               | kind number (string)                 | --                              | [18](18.md), [25](25.md), [72](72.md) |
 | `l`               | label, label namespace               | --                              | [32](32.md)                           |
 | `L`               | label namespace                      | --                              | [32](32.md)                           |