diff --git a/book/rhai/library/project.md b/book/rhai/library/project.md
index 8a335d8e..352b18b5 100644
--- a/book/rhai/library/project.md
+++ b/book/rhai/library/project.md
@@ -30,3 +30,11 @@ Returns the project prefix.
 ```ts
 HEMTT.project().prefix(); // "abe"
 ```
+
+## `mainprefix()`
+
+Returns the project main prefix, empty if not set.
+
+```ts
+HEMTT.project().mainprefix(); // "z"
+```