Deselecting Radio Buttons

[OneDay at A.Time]

Heelo All,

        It must be possible to deselect a radio button after selecting it. 
Would someone please tell me how?

Thanks for your help, Al

PMVar = IntVar()
PMBut = Radiobutton(root, text="Select for P.M.", value=2,/
                                         variable=PMVar, padx=50)
PMBut.grid(row=1, column=2,sticky=E)
PM = PMVar.get()