Step 1 of 5
Name the thing
An i tag holds a global id with a prefix that says what it is: isbn:…, geo:…, podcast:guid:…, a plain URL, bitcoin:tx:…
NIP-73
External Content IDs
DraftEvents
i and k tags point Nostr events at things outside Nostr (books, web pages, podcasts, places, blockchain transactions) so you can find every comment about them.
At a glance
- Status
- In use and still allowed to change.
- Tags
- i
- k
- Last changed
- Jun 13, 2026
Try it
Edit the form or the JSON: both stay in sync, the badge says whether the result is valid, and tapping any field explains it.
Not signed yet · Demo keys only. These personas' secret keys are public on purpose.
Explain
Select any field in the form or the JSON to see what it does.
How this NIP works
In the course
No course chapter covers this NIP yet. The editor above is the quickest way in.
Related NIPs
Used by
- NIP-22 Comment
NIP-22 comments use I/K (root) and i/k (parent) to comment on external content.
- NIP-25 Reactions
- NIP-84 Highlights
- NIP-85 Trusted Assertions
See also
- NIP-52 Calendar Events
Geohash ids work like NIP-52 location tags.
- NIP-24 Extra metadata fields and tags
Hashtag ids (#topic) relate to the t tags in NIP-24.
Specification
Specification text is quoted in English from the nostr-protocol/nips repository. Our explanations and the interface are translated. Source on GitHub at commit 0046368
Read the spec520 words, quoted from the repository at commit 0046368.
There are certain established global content identifiers such as Book ISBNs, Podcast GUIDs, and Movie ISANs that are useful to reference in nostr events so that clients can query all the events associated with these ids.
i tags are used for referencing these external content ids, with k tags representing the external content id kind so that clients can query all the events for a specific kind.
Supported IDs
| Type | i tag |
k tag |
|---|---|---|
| URLs | "<URL, normalized, no fragment>" |
"web" |
| Books | "isbn:<id, without hyphens>" |
"isbn" |
| Geohashes | "geo:<geohash, lowercase>" |
"geo" |
| Countries | "iso3166:<code, uppercase>" |
"iso3166" |
| Movies | "isan:<id, without version part>" |
"isan" |
| Papers | "doi:<id, lowercase>" |
"doi" |
| Hashtags | "#<topic, lowercase>" |
"#" |
| Podcast Feeds | "podcast:guid:<guid>" |
"podcast:guid" |
| Podcast Episodes | "podcast:item:guid:<guid>" |
"podcast:item:guid" |
| Podcast Publishers | "podcast:publisher:guid:<guid>" |
"podcast:publisher:guid" |
| Blockchain Transaction | "<blockchain>:[<chainId>:]tx:<txid, hex, lowercase>" |
"<blockchain>:tx" |
| Blockchain Address | "<blockchain>:[<chainId>:]address:<address>" |
"<blockchain>:address" |
Examples
Webpages
For the webpage "https://myblog.example.com/post/2012-03-27/hello-world" the "i" and "k" tags are:
[
["i", "https://myblog.example.com/post/2012-03-27/hello-world"],
["k", "web"]
]
Geohashes:
- Geohash:
["i", "geo:ezs42e44yx96"]- https://www.movable-type.co.uk/scripts/geohash.html
Geohashes are a geocoding system that encodes geographic locations into short strings of letters and digits. They MUST be lowercase.
Countries:
ISO 3166 codes can reference countries (ISO 3166-1 alpha-2) or subdivisions like states/provinces (ISO 3166-2).
- Country (Venezuela):
["i", "iso3166:VE"] - Subdivision (California, USA):
["i", "iso3166:US-CA"]
ISO 3166 codes MUST be uppercase. More info: https://en.wikipedia.org/wiki/ISO_3166
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 hyphens 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
Blockchain
<blockchain> can be any layer 1 chain (bitcoin, ethereum, solana, ...). If necessary (e.g. for ethereum), you can specify a <chainId>.
Bitcoin
bitcoin:address:<bech32, lowercase | base58, case sensitive>
bitcoin:tx:<txid, hex, lowercase>
E.g. https://blockstream.info/tx/a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d
[
["i", "bitcoin:tx:a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d"],
["k", "bitcoin:tx"]
]
E.g. https://blockstream.info/address/1HQ3Go3ggs8pFnXuHVHRytPCq5fGG8Hbhx
[
["i", "bitcoin:address:1HQ3Go3ggs8pFnXuHVHRytPCq5fGG8Hbhx"],
["k", "bitcoin:address"]
]
Ethereum (and other EVM chains)
ethereum:<chainId, integer>:tx:<txHash, hex, lowercase>
ethereum:<chainId, integer>:address:<hex, lowercase>
E.g. https://etherscan.io/address/0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045
[
["i", "ethereum:1:address:0xd8da6bf26964af9d7eed9e03e53415d37aa96045"],
["k", "ethereum:address"]
]
E.g. https://gnosisscan.io/tx/0x98f7812be496f97f80e2e98d66358d1fc733cf34176a8356d171ea7fbbe97ccd
[
["i", "ethereum:100:tx:0x98f7812be496f97f80e2e98d66358d1fc733cf34176a8356d171ea7fbbe97ccd"],
["k", "ethereum:tx"]
]
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]