It is commonly understood that the tzitz – the gold strip worn by the kohen gadol – was positioned on his forehead. This is the straightforward implication of the Torah’s command in Sefer Shemot (28:38), which instructs that the tzitz be worn “al meitzach Aharon” – “on Aharon’s forehead.” Indeed, Targum Onkelos translates this phrase to mean that the tzitz was placed “between the eyes.”
Interestingly, however, Tosefot in Masekhet Sukka (5a) cite a different opinion in the name of the Riva, who claimed that the tzitz was worn on the front part of the top of the head. The Riva advances this theory to explain why the Torah uses different terms in reference to the turban worn by ordinary kohanim and that worn by the kohen gadol. The turbans of the ordinary kohanim are called “migba’ot” (Shemot 28:40), whereas the kohen gadol’s turban is called a “mitznefet” (Shemot 28:39). The reason, the Riva explains, is because unlike the ordinary kohanim, the kohen gadol wore the tzitz, which occupied part of the head, thus necessitating a different, smaller kind of turban. If the tzitz was worn on the forehead, the Riva notes, then it would not affect the size of the kohen gadol’s turban. Therefore, it stands to reason that the tzitz was worn on the front of the head, thus requiring the kohen gadol to wear a smaller turban.
The Riva draws further proof from the Gemara’s comment in Masekhet Zevachim (19a), “The kohen gadol’s hair was visible between the tzitz and the turban, and it was there where he placed his tefillin.” The clear implication of the Gemara’s remark is that the tzitz occupied space on the head that could otherwise have been suitable for the tefillin, leaving only a small area of space on the head where the tefillin was worn. If the tzitz was worn on the forehead, then the kohen gadol’s head with respect to tefillin was no different from that of other kohanim, in that the tefillin simply needed to be placed in front of the turban. The fact that the presence of the tzitz created an especially small space for the tefillin would certainly seem to suggest that it was worn on top of the head, and not on the forehead.
As for the Torah’s requiring that the tzitz be worn on the “forehead,” the Riva draws a comparison to the Torah’s command in several places to wear tefillin “between your eyes” (“bein einekha”). Just as this phrase is understood to mean that the tefillin should be worn on the head, parallel to the space between the eyes, the command to wear the tzitz on the forehead likewise means that it should be worn above and parallel to the forehead, and not actually on the forehead.
Tosefot question this final point, noting that the Gemara in Masekhet Menachot (37b) establishes the placement of the tefillin on the head based on a different verse. The Torah in Sefer Devarim (14:1) forbids removing the hair “between your eyes,” and this clearly refers to the hair in the middle of the top of the head. Accordingly, the phrase “between your eyes” in reference to tefillin, too, refers not to the area between the eyes, but rather to the middle part of the top of the head, and this is where the tefillin is worn. This suggests that the placement of tefillin on the head is determined based on the specific phrase “bein eineikhem,” and thus we cannot extend this inference to the tzitz, which the Torah requires placing on the “meitzach.”