A separator is a simple widget that has no behaviour in response to the user and renders itself as a line (either horizontal or vertical.) It's used to separate parts of a GUI.
See Ttk, a part of Tk 8.5, for a core widget set that includes a separator. Its documentation can be found at http://www.tcl.tk/man/tcl8.5/TkCmd/ttk_separator.htm .
A separator can be rendered by a single frame suitably packed or grided:
proc separator {path} {
return [frame $path -relief groove -borderwidth 2 -width 2 -height 2]
}
# Horizontal separator
pack [separator .sep] -fill x