פירוש על מכות 3:10

מנין שהוא סמוך לארבעים – that causes to say afterwards “forty,” which is thirty nine. If the Torah had written: “Forty in number,” I would think that forty is the number. Now that it is written (Deuteronomy 25:2-3): “by count” (verse 2) and “Up to forty” (verse 3), it is the number that causes one to say after it “forty,” that is, “thirty-nine.”

Introduction Mishnah ten discusses the number of lashes a transgressor is to receive.

והיכן הוא לוקה את היתירה – that is not worthy to be divisible by three, for thirty-nine is worthy of being divided by three, one third in his front, and two thirds – one on this shoulder and one on the other shoulder. And this of forty – how would he flog it?

How many lashes is he given? Forty save one, as it says, “By number forty” (Deuteronomy 25:2-3) which means, a number close to forty. Rabbi Judah says: “He is given forty [lashes] in full.” And where does he receive the additional lash? Between his shoulders. Through a clever midrash our mishnah learns from Deuteronomy 25:2-3 that a person can receive up to 39 lashes for a single crime. The simple reading of those verses is actually that he can receive 40 lashes. Verse 3 states explicitly, “He may be given up to forty lashes.” The Rabbis derive 39 from the fact that verse 2 ends with the word “by number” and verse 3 with the word “forty”. The midrash is that the “number” is before “forty” and therefore he is lashed 39 times. Interestingly, the Rambam thinks that the midrash in our mishnah is not the derivation of the law but rather merely a Biblical support. The real reason that he only receives a maximum of 39 lashes is that if the person counting the lashes makes a mistake of one, he will still have only received 40 lashes. In this way the court will not violate the explicit prohibition in verse 3 not to give more than 40 lashes. Rabbi Judah reads the verse literally, and therefore does prescribe forty full lashes. In mishnah thirteen we will learn that the person receives a third of the lashes on his front and two-thirds on his back. The end of our mishnah asks where does he receive the fortieth lash, according to Rabbi Judah who says that he gets 40 full lashes. The mishnah teaches that the fortieth lash is between his shoulders.

בין כתפיו – But the Halakha is not according to Rabbi Yehuda.