Glynn, > Will this also work in 8.3? It's documented that way back to 8.0: http://www.tcl.tk/man/tcl8.0/TclLib/Interp.htm Plus I ran it in 8.3 while testing amongst running it in 8.4. What I don't understand is why grass 6 nvis runs fine on my computer with 8.4 and threads. --Cedric