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Hilchos Tefillin 36 (page 127)
מאות ס עד אות ק

The Beis Inside the Fay
Switching Between Beis Yosef Ksav and Velish
The Inverted Tzadi

The beis inside the fay
A fay is formed from an oversized chaf and an upside down inverted vav hanging inside it. L'chatchila, the interior area of the fay [the "empty space" inside it] should have the form of a beis . The outer rim of the chaf is formed the usual way, but inside the chaf there is a squared indentation in the line that connects the top and the bottom.  This indentation forms the heel of the beis . The inner edges of the chaf 's top, bottom and connector form the outer edges of the beis, and the edges of the hanging vav form the inner outline of the beis .
( משנת סופרים אות פ)
Switching between Beis Yosef ksav and Velish 
As far back as the times of the Rishonim, the Ashkenazim and Sefardim used different alphabets. The Asheknazi alphabet is referred to as ksav Beis Yosef and the Sefardi alphabet ksav Velish. According to many poskim, each ksav is kosher for the other group only b'dieved, but according to some poskim it is kosher l'chatchila. There is even an opinion which holds that it is possible to fulfill one's obligation by wearing tefillin in which the shel rosh and shel yad were written in different scripts. There is an opposing opinion which holds that Ashkenazim may not fulfill their obligation with ksav Velish, even b'dieved.
( משנת סופרים אות צ, ביאורים ומוספים דרשו, 2)
The inverted tzadi
The tzadi is formed from a yud and nun. In the standard Ashkenazi script, a regular yud is used.  In the Sefardi and Kabbalistic ksav, an inverted yud is used. Both forms of the letter can be found in ancient sources.
( משנת סופרים אות צ; ביאורים ומוספים דרשו, 2)

  • The basic components of the mem are a chaf connected to a vav. The lower right corner of the chaf is squared, and the bottom of the vav slants a little to the left.
  • The bottom of the nun should be wider than the top to prevent it from being read as a chaf or beis. The bottom should not be so wide that it will impede writing another letter next to it.
  • If a final nun follows a fay, it might seem like a zayin (so a word like pen can be read as paz). In this case, the final nun should be drawn a little longer. If it was not drawn longer, some poskim disqualify it and other poskim accept it.

  • The components of the kuf

  • 'Sheker (Falsehood) has no feet' and how that bears on halacha

  • The rule of mukaf gevil