diff --git a/README.md b/README.md
index 1dcb7bc..544f888 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,8 @@ On Mac, right click open to get past the security warning.
 For flatpak, Docker, and AppImage versions, refer to the wiki: https://github.com/ciromattia/kcc/wiki/Installation
 
 ## FAQ
+- How to make AZW3 instead of MOBI?
+  - The `.mobi` file generated by KCC is a dual filetype, it's both MOBI and AZW3. The file extension is `.mobi` for compatibility reasons.
 - [Windows 7 support](https://github.com/ciromattia/kcc/issues/678)
 - [Combine files/chapters](https://github.com/ciromattia/kcc/issues/612#issuecomment-2117985011)
 - [Flatpak mobi conversion stuck](https://github.com/ciromattia/kcc/wiki/Installation#linux)