Commentary for Deuteronomy 17:18

And that is that He commanded us that any king from our nation that sits on the royal throne should write a Torah scroll for himself, and that it not be separated from him. And that is His saying, "When he is seated on his royal throne, he shall have a copy of this Torah written for him" (Deuteronomy 17:18). And all of the regulations of this commandment have already been explained in Chapter 2 of Sanhedrin. (See Parashat Shoftim; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 7.)

והיה כשבתו AND IT SHALL BE WHEN HE SITTETH [UPON THE THRONE OF HIS KINGDOM] — if he acts thus (as prescribed in the previous verses) he is worthy that his kingdom should endure (Sifrei Devarim 160:1).

משנה התורה, my grandfather Rashi, explained this as meaning two separate Torah scrolls, [in line with the traditional interpretation of our sages in Sanhedrin 21, as distinct from Onkelos who derives the word משנה as related to שנן, (Deut. 6,7) to delve into the deeper meaning. Therefore,] Onkelos renders the word as פתשגן, a clearly worded interpretation, translation. [The king would have to have a version of the Torah at hand which a layman can understand without difficulty.]