Approximating Yud to Bayis of Tefillin

Approximating the knot of the Yud to the Bayis:[1]
One is to beware that the Yud of the knot[2] [of the Tefillin Shel Yad] does not move from the Bayis [of the Shel Yad].[3]  [It is thus to be constantly touching the Bayis of the Shel Yad. It is to touch the Bayis without any space in between.[4] It is to touch the actual Bayis and not just the Titura.[5] It is to touch the area of the Bayis that is facing the heart.[6] Many Poskim rule that this law applies even when one is not wearing the Tefillin.[7] Accordingly, those who are careful to have a plastic cube placed on the Tefillin Shel Yad for protection, must be careful to make a small hole in the cube to allow the Yud to touch the Bayis.[8]]

Summary:
One is to beware that the Yud of the Tefillin Shel Yad constantly touches the actual Bayis of the Shel Yad without any space in between. It is to touch the area of the Bayis that is facing the heart. Many Poskim rule that this law applies even when one is not wearing the Tefillin.

 Q&A

Where on the Bayis should the Yud to be touching?
Some Poskim[9] rule it suffices for the bottom of the Yud to touch the Titura [i.e. bottom section] of the Bayis and it is not necessary to have the top of the Yud touch the Bayis. Furthermore, one is specifically to do so in order so the remainder of the Yud touches one’s skin. However, majority of Poskim[10] rule the Yud touch the actual Ketzitza of the Bayis [i.e. the top square], and not the Titura alone and so is the final custom.

Which area of the Ketzitza: Some Poskim[11] rule the Yud is to be touching the area of the Bayis that faces the shoulder [i.e. the back of the Bayis] Other Poskim[12] however rule it is to touch the area of the Bayis that faces the heart [right side for righties] and so is the practical custom.

Is the Yud to be attached to the Bayis even when one is not wearing the Tefillin?
Many Poskim[13] rule that the Yud to be attached to the Bayis even when one is not wearing the Tefillin. Some Poskim[14] however rule it is only necessary for the Yud to be touching the Bayis while one is wearing it and fulfilling the Mitzvah, and so seems to be the position of the Chabad Rabbeim.[15]

If one wore the Tefillin without the Yud being attached to the Bayis, must he re-wear them?[16]
No.

How does one achieve that the Yud not move from the Bayis?
Tying a sinew to the Yud: Conforming with the above, some are careful to take a sinew string that attaches the Bayis to the Titura and use it to tie the Yud to the Bayis in a way that it remains steadfast attached to the Bayis without ability to move.[17] Those who do so must be careful to use the upper sinew and not the lower one in order to avoid an issue of Chatzitza.[18] Some Poskim[19] however negate this custom.
Other methods:[20] The Yud may not be glued to the Bayis. Some Batim makers form the hole by the Mabarta in a way that the Yud is squeezed inside and does not move. One can also bind the strap around the Yud and the Bayis.[21]

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[1] Admur 27/5; Siddur Admur “The Zohar states that the Yud is not to be moved from the Bayis.”; Michaber 27/2; See Piskeiy Teshuvos 27/6

[2] In Siddur Admur her states “Kesher Shel Yud” in Admur 27/5 and Michaber ibid he states “the Yud of the Kesher”

[3] The reason: This is due to the reason brought in the Zohar [Pinchas 3/236]. [Admur 27/5; M”B 27/10] The Zohar ibid was very stringent in this stating that one must be very careful that the Yud does not move from the Yad Tefillin in order so it does not create a separation, and one who does so is distanced from the pleasures of the next world. [Beis Yosef 27; Soles Belula 27/1; Kaf Hachaim 27/12]

[4] Ashel Avraham Butchach 27

Other opinions: Some Poskim rule it is valid if there is a very slight space in-between. [Teshuras Shaiy 2/93]

[5] See Q&A

[6] See Q&A

[7] See Q&A

[8] Igros Kodesh 12/433

[9] Avnei Nezer 14

[10] See Tzafnas Paneiach 3/160; Teshuras Shaiy 2/93; Shulchan Shlomo 27/3; Piskeiy Teshuvos 27 footnote 37

[11] Matzas Shmurim p. 64

[12] See Vayeishev Hayam 4/8 in name of Kisvei Arizal and Setimas Haposkim; Piskeiy Teshuvos ibid

[13] Implication of Zohar ibid; Mahariy Tzemach; Olas Tamid p. 57; Kesher Gudal 3/13; Shalmei Tzibur p. 40a; Siddur Beis Oved 29; Shaareiy Teshuvah 27/5; Chesed Lealafim 25/10; Ben Ish Chaiy Vayeira 15; M”B 27/10 “Some are careful”; Kaf Hachaim 27/12; Piskeiy Teshuvos 27/6

[14] Ashel Avraham Butchach 27

[15] See Igros Kodesh 12/433, printed in Shulchan Menachem 1/85, that the Rebbe Rayatz did not have a hole in the box that covered his Ketzitza of his Shel Yad, and that he did not wear the box during Davening; See also Yimei Melech 3/986 that the Rebbe Rayatz folded the straps over the Tefillin in a way that the Yud did not touch the Bayis and were behind the straps; See Siddur Raskin footnote 33 who infers this from Admur in Siddur

[16] Piskeiy Teshuvos 27/6

[17] Brought in Shaareiy Teshuvah 27/5; Kaf Hachaim 27/12; Rav SZ”A, brought in Piskeiy Tehsuvos ibid footnote 31, followed this custom

[18] Kaf Hachaim 27/12

[19] Shaareiy Teshuvah leaves this custom in question due to Chatzitza; M”B ibid negates the custom; Chazon Ish, brought in Piskeiy Tehsuvos ibid footnote 31, negates the custom as we find no source for allowing an additional sinew in the Rishonim or Achronim.

[20] See Piskeiy Teshuvos ibid footnotes 32-35

[21] See Zer Zahav of Mahriy Tzemach that so was custom of Rav Shmuel Vital; Kaf Hachaim 27/13 in name of Poskim that according to Kabala this binding is necessary, in addition to the need of approximating the Yud to the Bayis.