Eruvin, Chapter 2, Mishnah 1. This chapter introduces us to the concept of forming a partition with the most minimal walls possible. This was done especially around springs and water pits, in order to allow travelers, and primarily the pilgrims going up to Jerusalem, to water their animals along the way.
The problem was that these public roads were considered a public domain, while the pits, springs and wells were considered a private domain - and so drawing water and carrying it out from a private domain into a public domain would be forbidden. It was therefore necessary to surround the well with a minimal partition, and the Sages were lenient in this matter and permitted several very limited forms of partition, which we will now learn in the Mishnah.
The opinion of Rabbi Yehudah in the words of the Mishnah:
"Osin pasin l'vira'os" - one may make pasin, that is, boards, around those wells and springs. And how? "B'arba'ah diyumdin, nirin kishmonah" - four double boards, made as four right angles of ninety degrees, one at each corner, forming a square. They look like eight, because every right angle is made up of two boards, and four angles come to eight boards. Each one is ten tefachim high and an amah wide, so that each board extends an amah in each direction. "Divrei Rabbi Yehudah" - these are the words of Rabbi Yehudah.
The opinion of Rabbi Meir:
Rabbi Meir says: "Shemonah nirin kishneim asar" - eight boards are required, which look like twelve. How so? "Arba'ah diyumdin v'arba'ah peshutin" - the four double boards at the corners, which are eight boards, plus another four single boards standing between the right angles, a total of twelve. And each one of them is an amah wide and ten tefachim high.
As for their measurements, the Mishnah specifies:
"Govhan asarah tefachim" - their height is ten tefachim.
"V'rochban shishah" - their width is six tefachim, which is an amah.
"V'avyan kol shehu" - their thickness may be of any measure at all.
The size of the gap between the boards:
The dispute over whether there are eight boards or twelve depends on a further dispute: how large a gap is permitted between the boards, both between the corner boards and between the middle boards? On this the Mishnah says: "U'veineihen kimlo shtei revakos shel shalosh shalosh bakar, divrei Rabbi Meir" - and between them the space of two teams of three oxen each, these are the words of Rabbi Meir, while in contrast, "Rabbi Yehudah omer shel arba arba" - Rabbi Yehudah says teams of four each.
Rabbi Meir: the gap between the boards is the measure of two groups of three oxen, a total of six oxen. Each ox is an amah and two thirds of an amah wide, and six such oxen come to ten amos.
Rabbi Yehudah: two groups of four oxen each, and by this calculation the measure comes to thirteen and a third amos.
The Mishnah adds details regarding the width of these groups of cattle: "Kesuros v'lo mutaros" - tied to one another and not loose, and in this way an exact measurement is possible. And likewise "achas nichneses v'achas yotzes" - one team on its way in and one on its way out, and therefore more space is required. The total space, as the Gemara explains, is ten amos according to Rabbi Meir and thirteen and a third amos according to Rabbi Yehudah.
The connection between the two disputes:
The Gemara connects the two disputes - whether eight boards are required or twelve, and what the size of the gap is - in the following way: if the gap between the boards is ten amos or less, even Rabbi Meir, who required twelve boards, agrees that the middle boards are unnecessary, and the four right angles suffice. On the other hand, even Rabbi Yehudah agrees that if the gap is thirteen and a third amos or more, the additional boards must be added in order to reduce the gap, since this is the maximum gap permitted between the boards.
It emerges that the dispute is fundamentally not about whether twelve boards are required or eight, but about the range between ten amos and thirteen and a third amos: when the gap falls within this range, must the additional board be added in order to reduce it, or is that unnecessary?
In summary: in this Mishnah we learned the leniency of pasin around wells, which was intended to allow the pilgrims to water their animals even though the well is a private domain and the road is a public domain. We examined the opinion of Rabbi Yehudah - four diyumdin that look like eight, and the opinion of Rabbi Meir - eight that look like twelve, four diyumdin and four single boards, ten tefachim high, an amah wide and of any thickness at all. We also examined the size of the gap between them - ten amos according to Rabbi Meir and thirteen and a third amos according to Rabbi Yehudah - and the Gemara's explanation that the entire dispute concerns only the range between these two measures.