diff --git a/game/btl4main.cpp b/game/btl4main.cpp
index f4d007e..ce8abcb 100644
--- a/game/btl4main.cpp
+++ b/game/btl4main.cpp
@@ -394,6 +394,20 @@ static const char *kEnvironIniDefault =
 "# Phosphor colour of the mono MFDs, RRGGBB.  Default is a green tube.\n"
 "#BT_COCKPIT_TINT=21FF42\n"
 "\n"
+"# How far the lit RIO buttons reach out past the edge of a display, in pixels.\n"
+"# The pod's buttons were physical and backlit; on glass they are a thin bar\n"
+"# along the display edge, and a FLASHING one is easy to miss.  This is how much\n"
+"# more of that bar shows -- a LONGER bar, not a wider button: the button keeps\n"
+"# its width, the display picture still covers everything it did before, and the\n"
+"# whole display stays clickable.  Red (MFD) and yellow (map) buttons only; the\n"
+"# blue flight block has no glass to hide behind, so it never changes.\n"
+"#   10    the default\n"
+"#   0     back to the original thin bar\n"
+"#   1-200 anything in between\n"
+"# Give it a NUMBER -- a word like 'on' counts as 0 and turns it off.  In the\n"
+"# exploded view (BT_GLASS_PANELS) the windows grow to fit the longer buttons.\n"
+"#BT_RIO_PROTRUDE=10\n"
+"\n"
 "# ---- Display ----------------------------------------------------------------\n"
 "\n"
 "# The cockpit is fitted into the window at ONE uniform scale, centred, with\n"