diff --git a/boxes/__init__.py b/boxes/__init__.py
index 89b167e..464c3d2 100755
--- a/boxes/__init__.py
+++ b/boxes/__init__.py
@@ -240,7 +240,7 @@ class Boxes:
             help="format of resulting file")
         defaultgroup.add_argument(
             "--tabs", action="store", type=float, default=0.0,
-            help="width of tabs holding th parts in place (not supported everywhere)")
+            help="width of tabs holding th parts in place in mm (not supported everywhere)")
         defaultgroup.add_argument(
             "--debug", action="store", type=boolarg, default=False,
             help="print surrounding boxes for some structures")