2483e263_TCL_TK_Command_Reference_Guide__Configure_Option
[Resource Name] overRelief [Resource Class] OverRelief [Description] (Tk 8.4+) Alternative relief for when mouse cursor is over button. Not used when set to empty string (default). Options are: flat, raised, and sunken. [Configure Option] 
-overrelief type
TCL/TK Command Reference Guide