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Hilchos Tefillin 36 (page 124)
מ-אות ט עד אות כ

The Components of the Tes
The Shape of the Yud
The Kutzo Shel Yud

The components of the tes
The tes is composed a chaf combined with a zayin (crowned with three tagim). The left side of the roof should curve into the interior of the letter (but not bend into it like a fay). Care must be taken that the curved roof does not touch the inside walls of the letter. If the curve bends in like a fay, it should be repaired if the writing is for a sefer Torah. If it is for tefillin and mezuzos, however, where repairing would cause a problem with k'sidron, the letter is kosher bedieved. If the 'curved in' top touches the inside of the letter, it is pasul. If the top of the chaf section touches the zayin section, the poskim discuss whether the problem is limited to mukaf gevil (which is reparable even for tefillin) or whether, because the new form resembles a samach, erasing the connector is an irreparable problem of chok tochos.
( משנת סופרים אות ט וביה"ל ד"ה ולא; ביאורים ומוספים דרשו, 3)
The shape of the yud
The yud is composed of four parts: the roof, the leg, the tag and the ukatz. The roof is a short, thick line. It should not be made long or the letter will seem like a reish. The leg is shorter than that of a vav, but not too short, and is rounded on the right side. Care must be taken to keep the line short and straight, or the letter might look like a vav, chaf, nun, or samach. The ukatz is a short tip that extends from the lower left corner of the "roof" towards the left. This line must be shorter than the right leg in order to prevent the letter from looking like a ches. The ukatz should also be shorter than the tag. The tag is a short line which extends from the upper left corner of the roof. The line is longer than the lower ukatz, but not so long or tilted that it should make the letter seem like a lamed. According to some Acharonim, it is adequate for the ukatz and tag to be square corners and they do not need to extend out from the roof. According to this opinion, old sifrei Torah that do not meet the more stringent definition of ukatz and tag do not need to be repaired.
( משנת סופרים אות י; ביאורים ומוספים דרשו, 1, 2 ו־8)
The kutzo shel yud
The Gemara teaches that a yud without an ukatz is pasul. According to Rashi, the ukatz here refers to the right leg. According to Rabbeinu Tam and many more Rishonim, there would have been no need to mention the right leg specifically because without this leg the letter is not a yud. They hold that the Gemara is referring to the lower left tip as described above. There is also an opinion which defines the ukatz as the upper tag. The halachic conclusion is that the yud must have both the tag and the ukatz, and it is pasul if it lacks either one. A leg that was omitted cannot be added later.  Because a roof without a leg is not a yud, adding the leg [and thus completing the letter] afterwards is a problem of k'sidron. A missing ukatz may be added later since the general shape of the letter [roof with leg] was already written . The letter is kosher when it is missing a tag, but it must be repaired. According to some Aharonim, if a get was written without the ukatz on the yud, it can be kosher under certain circumstances.
( משנת סופרים אות י, וביה"ל ד"ה גם וד"ה יגרור; ביאורים ומוספים דרשו, 5 ו־9)

  • The left leg of the heh should be aligned with the left tip of the roof. The space between the left leg and the roof should not be greater than the thickness of the roof. It should not be too close to the roof; there should be some distance between them.
  • Chazal note that the vav in brisi shalom (in Parahas Pinchas) should be 'cut off.' There is a machlokes as to how this is applied.
  • According to most Rishonim, the ches is formed from two zayins connected with an arrowlike top called a chatoteres. The upper right corner of the zayin on the right should be rounded.

  • A chaf shaped like >

  • Similarities between a tes and a lamed

  • Forming the balance of the letters