The forms of the shins on the sides of the shel rosh
There are two shins on the sides of the tefillin shel rosh -- one on the right side of the bayis and one on the left side. These shins are required by halacha l'Moshe m'Sinai. The one to the wearer's right has three branches and the one to the wearer's left has four branches. If the shins were placed on the wrong sides, the tefillin are kosher b'dieved. If both are three-branched or both four-branched, the tefillin are pasul. There is a machlokes as to whether these shins must resemble Ashuris script and whether it must meet the other rules for writing stam, and the matter should be treated stringently.
(סעיף מב, ס"ק קצא, קצג ו־קצה, וביה"ל ד"ה שי"ן; ביאורים ומוספים דרשו, 232)