add_radio name onval offval variable value command
Multiple radio buttons are grouped together to form a set of which any one
button can be activated at any one time. The add_radio command adds
commands to menus in a similar fashion to the add_command command, but
has two additional arguments; variable and value.
Each radio button within a group uses the same variable with differing
values. When a radio button is selected the global Tcl variable is set
with the associated value and the command is executed. A useful tip is
that the contents of the variable may be passed to the command as an
argument using \$variable.
As each group can specify its own variable, multiple radio button groups are possible .