diff --git a/resources/definitions/fdmprinter.def.json b/resources/definitions/fdmprinter.def.json
index c930a624d0..de47e3ff86 100644
--- a/resources/definitions/fdmprinter.def.json
+++ b/resources/definitions/fdmprinter.def.json
@@ -461,7 +461,7 @@
                 "machine_head_with_fans_polygon":
                 {
                     "label": "Machine Head & Fan Polygon",
-                    "description": "The shape of the print head. These are coordinates relative to the position of the print head, which is usually the position of its first extruder. The dimensions left and in front of the print head must be negative coordinates.",
+                    "description": "The dimensions of the print head used to determine 'Safe Model Distance' when printing 'One at a Time'. These numbers relate to the centerline of the first extruder nozzle. Left of the nozzle is 'X Min' and must be negative.  Rear of the nozzle is 'Y Min' and must be negative.  X Max (right) and Y Max (front) are positive numbers.  Gantry height is the dimension from the build plate to the X gantry beam.",
                     "type": "polygon",
                     "default_value": [
                         [-20, 10],