diff --git a/docs/developer/src/templates.md b/docs/developer/src/templates.md
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--- a/docs/developer/src/templates.md
+++ b/docs/developer/src/templates.md
@@ -0,0 +1,29 @@
+Templates
+=========
+
+GNU social uses the [Twig template engine](https://twig.symfony.com/).
+When you handle a UI-related event, you add your own twig snippets either with
+`App\Util\Formatting::twigRenderFile` or `App\Util\Formatting::twigRenderString`.
+
+Example
+-------
+
+```php
+public function onAppendRightPanelBlock(array $vars, array &$res): bool
+{
+    if ($vars['path'] == 'attachment_show') {
+        $related_notes = DB::dql('select n from attachment_to_note an ' .
+    'join note n with n.id = an.note_id ' .
+    'where an.attachment_id = :attachment_id', ['attachment_id' => $vars['vars']['attachment_id']]);
+        $related_tags = DB::dql('select distinct t.tag ' .
+    'from attachment_to_note an join note_tag t with an.note_id = t.note_id ' .
+    'where an.attachment_id = :attachment_id', ['attachment_id' => $vars['vars']['attachment_id']]);
+        $res[] = Formatting::twigRenderFile('attachmentShowRelated/attachmentRelatedNotes.html.twig', ['related_notes' => $related_notes]);
+        $res[] = Formatting::twigRenderFile('attachmentShowRelated/attachmentRelatedTags.html.twig', ['related_tags' => $related_tags]);
+    }
+    return Event::next;
+}
+```
+
+Regarding using the Twig language, you can refer to
+[Twig Documentation](https://twig.symfony.com/doc/3.x/templates.html).
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