"M-x inferior-tcl" launches an embedded Tcl [REPL] in an Emacs buffer. "C-u M-x inferior-tcl" allows you to explictly specify which binary to run as the Tcl interpretter, which is handy for picking Tcl versions, or picking between wish and tclsh. For example:

"C-u M-x inferior-tcl", then keying "tclsh8.6" would run tclsh8.6 on a system containing a Tcl 8.6 installation with a tclsh named tclsh8.6. "C-u M-x inferior-tcl" then keying "wish" would load a REPL into an [Emacs] buffer, and also load display a Tk window, like running "wish" typically does. The benefit is that you get full editing features of Emacs.

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