Eruvin, Chapter 1, Mishnah 10. In the previous mishnah we learned how one can form a partition that permits carrying within it by means of horizontal ropes tied to stakes: as long as the ropes are within three tefachim of one another, the law of lavud applies to them, viewing the gaps between them as sealed, and therefore three relatively thin ropes suffice to create a partition ten tefachim high. Our mishnah deals with making a similar partition on the strength of the law of lavud, except that this time it is done with vertical stakes driven into the ground.
A Partition of Vertical Reeds:
The mishnah says: "Makifin bekanim" - one may enclose an area with reeds, by means of vertical stakes driven into the ground, "uvilvad shelo yehei bein kaneh lachaveiro sheloshah tefachim" - provided that there is no gap of three tefachim or more between one reed and the next. As long as they are within three tefachim, we view them as lavud, as a complete partition, and therefore they are considered a wall.
"Beshayara Dibru" - Rabbi Yehudah's View:
Regarding both of these types of partition - the horizontal ropes of the previous mishnah and the vertical reeds of our mishnah - the mishnah adds a qualification: "Beshayara dibru, divrei Rabbi Yehudah". In Rabbi Yehudah's view, the Sages were lenient to allow a partition made only of verticals or only of horizontals solely in the case of a caravan, that is, people who are traveling on the road, and in such a situation this suffices. And a caravan of people is no fewer than three. But for an individual it does not suffice.
The View of the Sages:
The Sages say: "Lo dibru beshayara ela behoveh" - the mention of a caravan is not a condition but a description of the usual circumstance, since a person typically needs an improvised partition of this sort precisely when he is traveling with a group of people and takes whatever is at hand in order to improvise a partition from it. In truth, however, even an individual may do so.
The View of Rabbi Yose bar Rabbi Yehudah:
The mishnah continues and brings a stricter view: "Kol mechitzah she'einah shel shesi veshel eirev einah mechitzah, divrei Rabbi Yose bar Rabbi Yehudah" - any partition that does not have both horizontals and verticals is not a partition. In this Rabbi Yose disagrees with his father: Rabbi Yehudah permitted relying on one of the two in the case of a caravan, whereas his son holds that even in a caravan, even for a group of people on the road, it is not permitted, and this is not a partition.
The Sages, on the other hand, say that one of the two suffices: one may make horizontal lines, and one may make vertical lines, and either one is valid. The Gemara clarifies that these Sages differ slightly from the Sages mentioned earlier: the first ones said that a caravan of people is not strictly necessary, and even for an individual it suffices, while these Sages say that one of the two types of partition suffices.
What is the difference between them?
The first Sages, although they permitted even an individual to use a partition that is only vertical or only horizontal, required that it be on the road - after the fact and in an improvised situation where the matter is difficult. But these Sages say that this makes no difference: whether a group of people or an individual, whether on the road or in a settled place, one may use either a vertical partition or a horizontal one.
The Measure of the Permitted Area:
The Gemara teaches a further limitation: an individual who makes such a partition is limited to a beis se'atayim - five thousand square amos and no more - and the same law applies to two people. But three people or more who make such a camp may take as much area as they need - for themselves, for their vessels and for their animals - provided that they do not have empty space greater than a beis se'atayim, greater than five thousand square amos.
Four Things Permitted to an Army Camp Going Out to War:
The mishnah continues and lists four things that were permitted to an army camp going out to war, for which special leniencies were made in halachah:
Wood: They may bring wood for burning or for other uses from anywhere, even if it has already been gathered and bundled by other people.
Washing the hands: They are exempt from washing the hands for hamotzi before the meal. However, the Gemara explains that the exemption was stated only regarding the washing before the meal; the washing after the meal, which involves a matter of health - out of concern for Sodomite salt, which can blind a person - still applies to them.
Demai: They are exempt from demai. They need not be concerned when buying produce from an am ha'aretz to separate terumas maaser, but may rely on the assumption that the am ha'aretz separated it.
Eruv: They need not be concerned about eruv chatzeros, about making an eruv in their shared areas. However, eruvei techumin - the prohibition against going beyond two thousand amos without an eruv - still applies to them.
In summary: In this mishnah we learned that one may enclose an area with vertical reeds on the strength of the law of lavud, provided that there are not three tefachim between one reed and the next. The Tannaim disagreed about the scope of the leniency: Rabbi Yehudah limits it to a caravan alone, which is no fewer than three people; the Sages hold that the caravan was mentioned only because it is the usual circumstance, and even an individual is included in the leniency; and Rabbi Yose bar Rabbi Yehudah is stricter and holds that there is no partition unless it has both horizontals and verticals together. We also examined the measure of a beis se'atayim for one or two people as opposed to three, the four leniencies stated for a camp going out to war - wood, washing the hands, demai and eruv chatzeros - and the two qualifications within them: mayim acharonim and eruvei techumin.