diff --git a/docs/Project_Structure.md b/docs/Project_Structure.md
index 91857e6..355a43e 100644
--- a/docs/Project_Structure.md
+++ b/docs/Project_Structure.md
@@ -43,7 +43,8 @@ Most of them are made in SVG.
 Except for illustrations, there must be an `fill="context-fill" fill-opacity="context-fill-opacity"` property to dynamically determine color and transparency.
 
 Icons are mainly [FirefoxUX/photon-icons](https://github.com/FirefoxUX/photon-icons)
-or [microsoft/fluentui-system-icons](https://github.com/microsoft/fluentui-system-icons).
+or [microsoft/fluentui-system-icons](https://github.com/microsoft/fluentui-system-icons).  
+Although not yet used, [tabler/tabler-icons](https://github.com/tabler/tabler-icons) and [feathericons/feather](https://github.com/feathericons/feather) can also be referred to.
 
 You can see more in the issue, [Unify icon design langauge #213](https://github.com/black7375/Firefox-UI-Fix/issues/213).