diff --git a/boxes/generators/gearbox.py b/boxes/generators/gearbox.py
index 7337e59..fabba7b 100644
--- a/boxes/generators/gearbox.py
+++ b/boxes/generators/gearbox.py
@@ -28,7 +28,7 @@ class GearBox(Boxes):
             help="number of teeth on outgoing shaft")
         self.argparser.add_argument(
             "--modulus",  action="store", type=float, default=3,
-            help="modulus of thetth in mm")
+            help="modulus of the theeth in mm")
         self.argparser.add_argument(
             "--shaft",  action="store", type=float, default=6.,
             help="diameter of the shaft")