frame .f
button .f.start -text START -font {-size -40} -background green -command "puts start"
button .f.stop  -text STOP  -font {-size -40} -background red   -command "puts stop"
button .exit -text EXIT -font {-size -60} -background yellow -command exit

pack .f -side top -expand true -fill both 
pack .exit -side bottom -expand true -fill x
pack .f.start -side left -expand true -fill both 
pack .f.stop  -side right -expand true -fill both