diff --git a/wiresheet.py b/wiresheet.py
index c2b7676..aa1aa17 100644
--- a/wiresheet.py
+++ b/wiresheet.py
@@ -575,13 +575,13 @@ def draw_knob(x, y, on_sheet, off_sheet, extra_tags=()):
                            outline="white", width=1, tags=tags)
 
     if on_sheet:
-        # A small white centre is what marks the far end as being on this sheet. It
-        # keeps its colour through selection, so the middle never fills in -- and on a
-        # half-and-half knob it sits in the wire-coloured half it belongs to.
-        cx = x - KNOB_RADIUS / 2 if off_sheet else x
+        # A small white centre is what marks the far end as being on this sheet. It keeps
+        # its colour through selection, so the middle never fills in. It stays centred
+        # even on a half-and-half knob: pushed into the wire-coloured half it would fill
+        # that half almost completely and leave no colour to read.
         canvas.create_oval(
-            cx - KNOB_CORE_RADIUS, y - KNOB_CORE_RADIUS,
-            cx + KNOB_CORE_RADIUS, y + KNOB_CORE_RADIUS,
+            x - KNOB_CORE_RADIUS, y - KNOB_CORE_RADIUS,
+            x + KNOB_CORE_RADIUS, y + KNOB_CORE_RADIUS,
             fill="white", outline="", tags=tags + ("link_knob_core",)
         )