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The mention of Lashon Ashuri, or Assyrian language, is referenced twice in the Tractate Megillah, in Megillah 17a:9 and Megillah 18a:23, where the Rabbi interchanges Ashuri with Hebrew. Hebrew is also referred to as Lashon Hakodesh, or Holy Tongue. The interchanging of Ashuri with Hebrew prompts the understanding that Ashuri, Hebrew, and Lashon Hakodesh are one and the same language.
Ktav Ashuri (Hebrew: כְּתָב אַשּׁוּרִי, ktav ashurí), or Assyrian script, is a traditional calligraphic form of the alphabet shared between Hebrew and Aramaic. Over some centuries, certain ornaments were simplified or removed for use outside traditional religious calligraphy, to become the modern print form of the Hebrew alphabet, which it most closely resembles.
Mention of the Ashuri script first appears in rabbinic writings of the Mishnaic and Talmudic periods, referring to the formal script used in certain Jewish ceremonial items, such as sifrei Torah, tefillin, and mezuzot. Also sometimes called the "square" script, the term is used to distinguish the Ashuri script from the Paleo-Hebrew alphabet.
In the Tractate Sanhedrin 22a:2 and 22a:4 it states,
למה נקרא אשורית שעלה עמהם מאשור (Why is Ashuri referred to as Ashuri?)
The baraita continues: Why is this script called Ashurit? Because it ascended with the Jewish people from Ashur when they returned from their exile in Babylonia.
למה נקרא שמה אשורית שמאושרת בכתב (Why is Ashuri approved in writing?)
The baraita continues: If this script predates the exile to Babylonia, why is it called Ashurit? Because it is meusheret, beautiful and straight, in script.
The Talmud states that Ashuri was authorized (meusheret) in writing, and that the script ascended with the Jewish people from Ashur, upon their return from the Babylonian exile. This is how the Assyrian script became in use for the Jewish people. The reference of the script as beautiful and straight is a subjective understanding of the Rabbi, in the Talmud itself it is only asked why the Ashuri in the Tractate Megillah is referred to as Ashuri, to which the baraita continues because it was authorized for writing, ascending from Ashur.
There are many rules concerning the proper formation of letters if the written text is to be valid for religious purposes.
Ashkenazi, Sephardi, Mizrahi, and Yemenite Jews each have their own calligraphic tradition regarding certain details of how each letter is formed, although the overall shape is similar. Generally, while each tradition favors their own calligraphic style, none consider the other traditions passul (invalid) for Torah scrolls or any other ritually used scroll or parchment.
Samaritans maintain a calligraphic tradition different from the Ashuri script, using instead the Paleo-Hebrew alphabet they employ for their scriptures.