A Tk toolkit extension, including widgets, geometry managers, etc BLT is an extension to the Tk toolkit. BLT's most useful feature is the provision of more widgets for Tk, but it also provides more geometry managers and miscellaneous other commands. Note that you won't need to do any patching of the Tcl or Tk source files to use BLT, but you will need to have Tcl/Tk installed in order to use BLT. If you are programming with the Tk toolkit, you should install BLT. You will need to have Tcl/Tk installed.