diff --git a/readme.md b/readme.md
index 074efd3..07e6b85 100644
--- a/readme.md
+++ b/readme.md
@@ -5055,7 +5055,7 @@ a cylinder. To get around this a colour can be passed to thread that is used to
 
 Making the ends requires a CGAL intersection, which make threads relatively slow. For this reason they are generally disabled when using the GUI but can
 be enabled by setting ```$show_threads``` to ```true```. When the tests are run, by default, threads are enabled only for things that feature them like screws.
-This behaviour can be changed by setting a ```SHOW_THREADS``` environment variable to ```false``` to disable all threads and ```true``` to enable all threads.
+This behaviour can be changed by setting a ```NOPSCADLIB_SHOW_THREADS``` environment variable to ```false``` to disable all threads and ```true``` to enable all threads.
 The same variable also affects the generation of assembly diagrams.
 
 Threads obey the $fn, $fa, $fs variables.