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Hilchos Tefillin 33 (page 115)
מסעיף ה עד תחילת סימן לד

Kshira Tama - Proper Knotting
Using a Torn Retzua
Retzua Which Tore in an Area Beyond the Required Length

Kshira tama - proper knotting
The Torah's word uk'shartam can be understood as the phrase uk'shar tam, meaning that the retzua required for tefillin should be prepared properly. According to many poskim, this is accomplished by having only one knot in the area of the retzua that is required for the mitzva. Other poskim explain that the phrase requires that the retzua be complete and untorn. How long must the retzua be? According to some, on the shel rosh it should be just long enough to wrap around the head and knot it. Others add two tefachim to that length, and a third opinion holds that the right retzua should reach the navel and the left one should reach the chest. The retzua of the shel yad must be long enough to tie the bayis to the arm, and should preferably extend far enough to wrap around the middle finger.
( סעיף ה, וס"ק כו, כח ו־כט; ביאורים ומוספים דרשו, 24 ו־26)

Using a torn retzua
A retzua which tore somewhere along the length that is required for it to be kosher [i.e. enough to wrap around the head] should not be fixed by tying the pieces together or joining them with string. The poskim debate whether it can be sewn from below in a manner that the stitches are not visible from the top. When no other retzuos are available, stitched retzuos may be used without a beracha. According to some poskim, there cannot be any stitching on the part that wraps around the finger. Another opinion does not allow any stitching between the knot on the arm and the part wrapped around the finger because that section is essential to holding the top knot in place.
( סעיף ה, ס"ק כה, כו, כח ו־כט, וביה"ל ד"ה אם; ביאורים ומוספים דרשו, 27)

A retzua which tore in an area beyond its required length
According to the opinion above, there cannot be any repairs to the section of retzua that wraps around the head. If there is a repair in a section which might not be required for the kashrus of the retzua and no other retzua is available, the tefillin may be worn with a beracha. A retzua of a shel rosh which tore below the necessary section does not need to be repaired. A torn retzua of a shel yad, however, should be repaired to allow for wrapping the retzua around the arm seven times. Ideally, it should be sewn from underneath with sinews, in a manner not visible from the outside. It is permissible, however, to sew it in any manner -- including stitching visible from the outside and knotting the two ends together.
( סעיף ה, ס"ק כט, וביה"ל ד"ה אם וד"ה אין)

  • A halacha l'Moshe m'Sinai teaches that the retzuos must be black. Ideally, they should be deep raven-black, but light black is kosher b'dieved.
  • According to the Rambam, it is considered a beautification of the mitzva if the underside of the retzuos is painted to match the color of the batim. According to the Rambam, painting the underside of the retzuos is not obligatory, but recommended.
  • The batim, titura, ma'avarta and shins must be made by an adult Jewish male lishma.

  • The order of the parshios according to Rashi and Rabbeinu Tam

  • Wearing Rabbeinu Tam tefillin

  • Wearing both tefillin at once