why is strike-through button not appearing on toolbar?

I downloaded Standard version.  With the following config code, every item listed, except for strikethrough, appears on the toolbar.  I have also tried strikeThrough, strikethrough, Strikethrough.

 CKEDITOR.replace('editor1');

CKEDITOR.config.toolbar =  [                 

                [ 'Cut', 'Copy', 'PasteText'],  '-' ,[ 'Undo', 'Redo' ],     [ 'Bold', 'Italic', 'StrikeThrough' ] ,  ['NumberedList','BulletedList']        
               ];