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diff --git a/91.md b/91.md
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@@ -0,0 +1,18 @@
+# NIP-91: AND Operator in Filters
+
+Enable `AND` within a single tag filter by using an `&` modifier in filters for indexable tags.
+
+```
+filters: {
+  "kinds": [1],
+  "&t": ["meme", "cat"],
+  "#t": ["black", "white"]
+}
+// returns kind `1` events with `t` tags that have both "meme" and "cat" that have the tag "black" or "white"
+```
+
+## Rules
+
+- `AND` **MUST** take precedence over `OR`
+- Tag values used in `AND` **SHOULD NOT** be used in standard `OR` tags [`#`]
+- Any tag value used in `AND` **SHOULD** be ignored in `OR` 
diff --git a/README.md b/README.md
index c094d47..e8a21f0 100644
--- a/README.md
+++ b/README.md
@@ -98,6 +98,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 - [NIP-88: Polls](88.md)
 - [NIP-89: Recommended Application Handlers](89.md)
 - [NIP-90: Data Vending Machines](90.md)
+- [NIP-91: AND filters](91.md)
 - [NIP-92: Media Attachments](92.md)
 - [NIP-94: File Metadata](94.md)
 - [NIP-96: HTTP File Storage Integration](96.md) --- **unrecommended**: replaced by blossom APIs
@@ -316,6 +317,17 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 [marmot]: https://github.com/marmot-protocol/marmot
 
 
+## Filters 
+
+| kind            | description                | NIP                      |
+| --------------- | -------------------------- | ------------------------ |
+| `authors`       | by pubkey                  | [01](01.md)              |
+| `ids`           | by ids                     | [01](01.md)              |
+| `kinds`         | by kinds                   | [01](01.md)              |
+| `#{}`           | by indexable tag (OR)      | [01](01.md)              |
+| `&{}`           | by indexable tag (AND)     | [91](91.md)              |
+| `search`        | by search times            | [50](50.md)              |
+
 ## Message types
 
 ### Client to Relay