Having the hairs protrude from the Bayis of the Shel Rosh:[1] The sages of the Zohar warned that part of this this hair is to protrude outside of the Bayis Shel Rosh.[2] It is to protrude less than the length of a barley [i.e. less than 1 centimeter].[3] It is disputed[4] as to from between which Parshiyos it is to protrude, whether from the Parsha of Kadesh or Vehaya Im Shamoa, and practically it is to protrude from the Parsha of Vehaya Im Shamoa, towards the side of Kadesh.[5] [It is due to this that one can differentiate between the Tefillin Shel Rosh of Rashi versus Rabbeinu Tam, as by Rashi the hair protrudes towards the far right of the Bayis, while by Rabbeinu Tam it protrudes in the middle being that by the Tefillin of Rabbeinu Tam the Parsha of Vehaya is in the middle and the hair protrudes from between the Parsha of Vehaya Im Shamoa and Vehaya Ki Yeviacha, which is exactly in the center of the Tefillin.[6] It suffices to have a single hair protrude[7], although some are accustomed to have many hairs protrude. Some argue that one should have at least one hair protrude from both the internal and external hair that is bound around the Parsha. The hair should remain visible and therefore should not be dyed with the ink. If the hair is not protruding, then the Tefillin is to be opened and have it fixed.[8] When checking the Tefillin one is to be particular that each parchment and hair is returned to its Parsha, and is certainly not to mix up the hairs and parchments of the Shel Yad and Shel Rosh, or Rashi and Rabbeinu Tam.[9]]

[1] Admur 32:69; Michaber 32:44; See Piskeiy Teshuvos 32:69

[2] The hole from which it protrudes: See Tefila Lemoshe 2:3 and Piskeiy Teshuvos ibid footnote 507 regarding if a special hole is to be made for this hair to protrude from, or if one should use one of the holes made for the sewing.

[3] Admur ibid; M”A 32:61 in name of Zohar; Hagahos Rav Akiva Eiger in name of Neziros Shimshon; Mur Uketiza 32; Piskeiy Teshuvos ibid footnote 508

[4] Tolas Yaakov [Parshas Kadesh] versus Shelah in next footnote

[5] Admur ibid; M”A 32:61 in name of Shelah Miseches Chulin

[6] Piskeiy Teshuvos ibid

[7] Zohar, brought in M”A 32:61

[8] See Piskeiy Teshuvos ibid for the sources of all the above statements

[9] See Kol Yaakov 32:208; Or Letziyon 2:44; Piskeiy Teshuvos ibid