What did he place on the Mitznefes? Why can it not have been the Tzitz?
Rashi: It was the Techeiles thread that he placed on the Mitznefes as a sort of crown. 1 It cannot have been the Tzitz, because the Kohen Gadol wore his Tefilin 2 between the Tzitz (which was placed on his forehead) and the Mitznefes. 3