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Hilchos Tefillin 33 (page 119)
מסעיף ד עד תחילת סימן לו

Exchanging the Components of Rashi and Rebbeinu Tam Tefillin
Not Wearing Rebbeinu Tam Tefillin on Chol Hamoed
The Number of Lines for the Tefillin

Exchanging the components of Rashi and Rabbeinu Tam tefillin
For people who use both Rashi and Rabbeinu Tam tefillin, one of the tefillin is always understood to be kosher tefillin with kedusha while the other is not. Since the casing and accessories of something with kedusha may not be used for non- kedusha items, the casing for one type of tefillin should not be used for the other type and they should be clearly marked to prevent mix-ups. It is customary to store the tefillin in separate bags as well. Some are lenient and permit using retzuos from Rabbeinu Tam tefillin for Rashi tefillin when other retzuos are not available since the halacha follows Rashi's opinion.
( סימן לד, סעיף ד וס"ק כ)

Not wearing Rabbeinu Tam tefillin on Chol Hamoed
We have previously learned that there is a machlokes between the Rishonim regarding tefillin on Chol Hamoed and there are differing customs about whether or not to wear them. Even those who wear tefillin on Chol Hamoed do not wear the tefillin of Rabbeinu Tam, in deference to the opinion of the Zohar which is stringent about wearing tefillin on Chol Hamoed. Additionally, there is a sfek sfeika (double safek) that pertains to wearing them:  1) uncertainty as to whether the halacha is in accordance with Rabbeinu Tam and 2) uncertainty as to whether tefillin are required on Chol Hamoed.
( סימן לד, ס"ק כ; ביאורים ומוספים דרשו, 31)

The number of lines for the tefillin
There is an ancient custom among sofrim to spread the writing of the parsha of the shel yad across seven lines and the writing of the shel rosh across four lines. The tefillin are kosher if they were written on a different number of lines. If the dimensions of the klaf would require the sofer to employ a small script that might jeopardize the validity of the tefillin, it is preferable to use a larger script and alter the number of lines.
( סימן לה, סעיף א, וס"ק א-ג)


  • The Acharonim note that today, when many people wear Rabbeinu Tam tefillin, it would not seem arrogant for an individual to wear them and it is even praiseworthy for someone do so. According to the Gra, however, it is not necessary to ever wear Rabbeinu Tam tefillin because the halacha follows Rashi.
  • Those who don Rabbeinu Tam tefillin should not remove their Rashi tefillin before Uva L'tzion. If they don the Rabbeinu Tam tefillin during chazarash hashatz, they may be distracted from listening to chazaras hashatz. Someone whose time is limited may be permitted to don them during chazaras hashatz if he has no other time to put them on.
  • Someone who wears both Rashi and Rabbeinu Tam tefillin must bear in mind to fulfill the obligation with only the true tefillin and view the other pair as non-mitzva. If he wishes to perform the mitzva with both pairs he may be in violation of both ba'al tosif and ba'al tigra.

  • Ashkenaz (Beis Yosef) script versus Sefard script

  • How the heh and kuf differ from other letters

  • The length of the tagim