Rashi on Exodus 29:6/רש"י על שמות כ״ט:ו׳

1נזר הקדש THE DIADEM OF HOLINESS — This refers to the Plate.

2על המצנפת UPON THE MITRE, as I have explained above (Exodus 28:37) — by means of the middle string and the two strings at its (the Plate’s) ends which were, the three of them, tied together behind his neck he placed it upon the mitre as a kind of helmet.