MB 14: Have the custom to crown - in tefillin other letters besides these, and they are listed in the Tur in the name of the Shimusha Raba and in the name of the Rambam [= Maimonedes]. These crowns are bigger [designs], but they should be thin so that the [form of the] letter is not ruined by them. One must be extra carefule with the crowns on a "vav" or "yud", because these letters can be easily changed by [adding crowns] [ since they are themselves very small].
MB 15: It is not "posul" - But the BaCH invalidates this. Additionally, the GR"A, in his commentary, logically justifies each opinion [in this argument], and brings many authorities who are strict about this [like the BaCH]. Therefore, it is certainly proper to suspect [that the BaCH is correct] and to repair it afterwards [if you find that crowns have been omitted]. Repair is effective, even in tefillin and mezuzahs, and this is not considered writing out of order, because even without the crowns the [basic] form of the letter is present. Know, additionally, that the Shulchan Aruch [who is lenient on this issue] is referring to a case where one made the top of the letter properly but left out the crowns; but if he also made the top of the letter round [instead of square, so that not only are there no crowns, but there are not even crown-like corners - AB], it seems from the Beis Yosef that one cannot be lenient even post facto according to the interpretation of the ReE"M. Practically, [this question] requires contemplation, because it seems that according to the Rambam, one can always be lenient [i.e. even if the top of the letter isn't squared]. (See the commentary of the GR"A, [who explains that] according to the Rambam the Gemara [which mentions crowns] is not dealing with tefillin at all, only with mezuzahs. [And it is not dealing with] rounding or squaring [the top of the letter], but with actual crowns and only "l'chatchila".) It is written in the work Iggeres HaTiyul that "shaatnez" are the letters of "SaTaN" "AZ" [= Brazen], which are the great accusers. This is also the secret of "shaatnez getz", for "getz" is also the name of an accuser, and the crowns on them are like a sword and spear [with which] to be saved from them.
Assaf Bednarsh bednarsh@yu1.yu.edu