Version 4 of Sew what - a discussion of Tcl and threads

Purpose: a place to share your wisdom, confusion, hard won knowledge, ... regarding the use of Tcl and threads.

What are threads?
How are threads in Tcl different than people expect?
How do I tell if I can use threads in Tcl? Before being able to use threads in Tcl, you have to specifically specify a configuration time flag. The default is to not build with threads. Unfortunately, the last I checked, the configuration with the thread flags built libraries and executables with the same name as configuring without the flag; this means that you cannot tell simply by file names whether your Tcl has threads or not.
How can I tell whether my Tcl interpreter has been compiled with threads?

  # Check for threaded Tcl
  proc is_threaded {} {
     expr {[info exists ::tcl_platform(threaded)] && $::tcl_platform(threaded)}
  }