Question Received by email

A Chabad Rabbi asked me to post this:

The Alter Rebbe writes in the Piskey Hasiddur as we know, in Hilchos Tefilin that the Shel Rosh has to be between the eyes “Mamesh”. Meaning that we need to make sure its align as perfect as we can,  due to the reason he gives,  that according to Rashi’s shita,  two parshiyos are on the right and two on the left, and based on the Rosh’s pirush -  its on one’s head that two have to be on one’s right side of the head and the other two on the left side of the head etc.  (checkout the leshon in the siddur below) .

"ואח"כ יניח את של ראש בגובה הראש ויזהר שיהא באמצע רוחב הראש ממש שהרי אמרו 'קדש' 'והי' כי יביאך' מימין ו'שמע' 'והי' אם שמוע' משמאל, ובתפילין דרש"י שהפרשיות כסדרן אין בין ימין לשמאל אלא משהו דהיינו החריץ שבין והי' כי יביאך לשמע, על כן צריך ליזהר בזה מאוד".

However, the Alter Rebbe writes in shulchan Aruch siman 32 seif Nun Tes , (based on the Baruch She’amar)  that the  Bayis of shel Rosh only lachatchila needs that all four compartments /batim should be equally in size/width,

ושל ראש יהיה כל הד' בתים יחד מרובעין ולא כל אחד בפני עצמו -ולכתחלה יעשה כל הד' בתים שוין שלא יהא אחד רחב מחבירו

So my question is, if the batim machers are not  particular about this,  and each compartment of the Shel Rosh made, may be little bit off and are not identical in width,  how is it possible that the Shel Rosh will ever be in the middle of the head where two parshiyos are on the right and two on the left?  (based on the above words of the Alter Rebbe in the Piskey Hasidur) If one bayis is little small and the other slightly wider,  the two parshiyos on either side will never be in the middle

באמצע רוחב הראש ממש

Can you please clarify this klutz kasha for me?