diff --git a/attachments.scad b/attachments.scad
index 47d409c..23f54d7 100644
--- a/attachments.scad
+++ b/attachments.scad
@@ -2278,6 +2278,14 @@ module corner_profile(corners=CORNERS_ALL, except=[], r, d, convexity=10) {
 //   * Rotates this part so it's anchor direction vector exactly opposes the parent's anchor direction vector.
 //   * Rotates this part so it's anchor spin matches the parent's anchor spin.
 //   .
+//   This module is also responsible for handing coloring of objects with {{recolor()}} and {{color_this()}}, and
+//   it is responsible for processing tags and determining whether the object should
+//   display or not in the current context.  The determination to display the attachable object
+//   occurs in this module, which means that an object which does not display (e.g. a "remove" tagged object
+//   inside {{diff()}} cannot have internal {{tag()}} calls that change its tags and cause submodel
+//   portions to display: the entire child simply does not run.  
+
+
 //   For a step-by-step explanation of attachments, see the [Attachments Tutorial](Tutorial-Attachments).
 //
 // Arguments:
diff --git a/gears.scad b/gears.scad
index befc19a..eab8a1b 100644
--- a/gears.scad
+++ b/gears.scad
@@ -72,6 +72,7 @@ function _inherit_gear_thickness(thickness) =
 //   this section provides the minimal information needed for gear making.  If you want more information about the
 //   details of gears, consult the references below, which are the ones that we consulted when writing the library code.
 //   - Tec Science
+//       * [Involute Gears](https://www.tec-science.com/mechanical-power-transmission/involute-gear/geometry-of-involute-gears/)
 //       * [Gear engagement](https://www.tec-science.com/mechanical-power-transmission/involute-gear/meshing-line-action-contact-pitch-circle-law/)
 //       * [Gears meshing with racks](https://www.tec-science.com/mechanical-power-transmission/involute-gear/rack-meshing/)
 //       * [Gear undercutting](https://www.tec-science.com/mechanical-power-transmission/involute-gear/undercut/)
@@ -2567,7 +2568,7 @@ function _gear_tooth_profile(
     rrad = _root_radius(circ_pitch, teeth, clearance, helical=helical, profile_shift=profile_shift, internal=internal),
 
     srad = max(rrad,brad),
-    tthick = circ_pitch/PI / cos(helical) * (PI/2 + 2*profile_shift * tan(pressure_angle)) - backlash,
+    tthick = circ_pitch/PI / cos(helical) * (PI/2 + 2*profile_shift * tan(pressure_angle)) + (internal?backlash:-backlash),
     tang = tthick / prad / 2 * 180 / PI,
 
     // Generate a lookup table for the involute curve angles, by radius