diff --git a/readme.md b/readme.md
index 245f382..de48258 100644
--- a/readme.md
+++ b/readme.md
@@ -5546,6 +5546,8 @@ specify a chamfer angle.
 Threads are by default solid, so the male version is wrapped around a cylinder and the female inside a tube. This can be suppressed to just get the helix, for
 example to make a printed pot with a screw top lid.
 
+A left hand thread can be made by using mirror([0,1]).
+
 Threads with a typical 60 degree angle appear too bright with OpenSCAD's primitive lighting model as they face towards the lights more than the top and sides of
 a cylinder. To get around this a colour can be passed to thread that is used to colour the cylinder and then toned down to colour the helix.
 
diff --git a/tests/png/thread.png b/tests/png/thread.png
index 8e2a221..cd5010d 100644
Binary files a/tests/png/thread.png and b/tests/png/thread.png differ
diff --git a/tests/thread.scad b/tests/thread.scad
index 6cceecc..c00cd87 100644
--- a/tests/thread.scad
+++ b/tests/thread.scad
@@ -26,27 +26,27 @@ profile = thread_profile(pitch / 2, pitch * 0.366, 30);
 
 module threads()
     for(female = [false, true]) translate([0, female ? -20 : 0]) {
-    length = female ? 8  : 40;
-    dia = female ? 8 : 8 - pitch;
-    colour = female ? brass : silver;
+        length = female ? 8  : 40;
+        dia = female ? 8 : 8 - pitch;
+        colour = female ? brass : silver;
 
-    thread(dia, starts * pitch, length, profile, starts = starts, top = 45, bot = 45, female = female, colour = colour);
+        thread(dia, starts * pitch, length, profile, starts = starts, top = 45, bot = 45, female = female, colour = colour);
 
-    color(colour)
-        translate([20, 0])
-            thread(dia, starts * pitch, length, profile, starts = starts, top = 0, bot = 0, female = female);
+        color(colour)
+            translate([20, 0])
+                thread(dia, starts * pitch, length, profile, starts = starts, top = 0, bot = 0, female = female);
 
-    translate([40, 0])
-        thread(dia, starts * pitch, length, profile, starts = starts, top = -1, bot = -1, female = female, colour = colour);
+        translate([40, 0])
+            thread(dia, starts * pitch, length, profile, starts = starts, top = -1, bot = -1, female = female, colour = colour);
 
-    color(colour)
-        translate([60, 0])
-            thread(dia, 2 * pitch, length, profile, starts = 2, top = -1, bot = -1, female = female);
+        color(colour)
+            translate([60, 0])
+                thread(dia, 2 * pitch, length, profile, starts = 2, top = -1, bot = -1, female = female);
 
-    color(colour)
-        translate([80, 0])
-            thread(dia, pitch, length, profile, starts = 1, top = -1, bot = -1, female = female);
-}
+        color(colour)
+            translate([80, 0])
+                thread(dia, pitch, length, profile, starts = 1, top = -1, bot = -1, female = female);
+    }
 
 let($show_threads = true)
     threads();
diff --git a/utils/thread.scad b/utils/thread.scad
index fc14fc7..4b1c360 100644
--- a/utils/thread.scad
+++ b/utils/thread.scad
@@ -26,6 +26,8 @@
 //! Threads are by default solid, so the male version is wrapped around a cylinder and the female inside a tube. This can be suppressed to just get the helix, for
 //! example to make a printed pot with a screw top lid.
 //!
+//! A left hand thread can be made by using mirror([0,1]).
+//!
 //! Threads with a typical 60 degree angle appear too bright with OpenSCAD's primitive lighting model as they face towards the lights more than the top and sides of
 //! a cylinder. To get around this a colour can be passed to thread that is used to colour the cylinder and then toned down to colour the helix.
 //!
@@ -61,10 +63,12 @@ module thread(dia, pitch, length, profile, center = true, top = -1, bot = -1, st
     // Extract some properties from the profile, perhaps they should be stored in it.
     //
     h = max([for(p = sprofile) p.y]);
-    maxx = max([for(p = sprofile) p.x]);
-    minx = min([for(p = sprofile) p.x]);
-    crest_xmax = max([for(p = sprofile) if(p.x != maxx) p.x]);
-    crest_xmin = min([for(p = sprofile) if(p.x != minx) p.x]);
+    xs = [for(p = sprofile) p.x];
+    maxx = max(xs);
+    minx = min(xs);
+    crest_xs = [for(p = sprofile) if(p.y == h) p.x];
+    crest_xmax = max(crest_xs);
+    crest_xmin = min(crest_xs);
     //
     // If the ends don't taper we need an extra half turn past the ends to be cropped horizontally.
     //
@@ -129,11 +133,13 @@ module thread(dia, pitch, length, profile, center = true, top = -1, bot = -1, st
                     render() intersection() {
                         polyhedron(points, ends_faces);
 
-                        len = length - 2 * eps;
+                        shorten = !is_undef(colour);
+                        len = shorten ? length - 2 * eps : length;
+                        offset = shorten ? eps : 0;
                         rotate_extrude()
                             if(female) {
                                 difference() {
-                                    translate([0, eps])
+                                    translate([0, offset])
                                         square([r + h + overlap, len]);
 
                                     if(top_chamfer_h)
@@ -146,7 +152,7 @@ module thread(dia, pitch, length, profile, center = true, top = -1, bot = -1, st
                             else
                                 difference() {
                                     hull() {
-                                        translate([0, eps])
+                                        translate([0, offset])
                                             square([r, len]);
 
                                         translate([0, bot_chamfer_h])