If a caravan camped in a valley, i.e., an open space not enclosed by walls, and the travelers enclosed their camp with partitions made of the animals’ equipment, e.g., saddles and the like, one may carry inside the enclosed area, provided that the resultant partition will be a fence ten handbreadths high, and that there will not be breaches in the partition greater than the built segment. Any breach that is approximately ten cubits wide is permitted and does not invalidate the partition because it is considered like an entrance. However, if one of the breaches is greater than ten cubits, it is prohibited to carry anywhere in the enclosed area.
(ט) מַקִּיפִין שְׁלֹשָׁה חֲבָלִים, זֶה לְמַעְלָה מִזֶּה וְזֶה לְמַעְלָה מִזֶּה, וּבִלְבַד שֶׁלֹּא יְהֵא בֵּין חֶבֶל לַחֲבֵרוֹ שְׁלֹשָׁה טְפָחִים. שִׁעוּר חֲבָלִים וְעָבְיָן, יָתֵר עַל טֶפַח, כְּדֵי שֶׁיְּהֵא הַכֹּל עֲשָׂרָה טְפָחִים:
(9) If a caravan is camped in a field, and the travelers seek to construct partitions to render the area fit for one to carry within it on Shabbat, one surrounds the area with three ropes, one above another, and a third one above the other two. One is permitted to carry within the circumscribed area provided that there will not be a gap of three handbreadths between one rope and the next. The measure of the ropes and their combined thickness must be greater than a handbreadth, so that the entire partition, consisting of three ropes and the empty spaces between them, will be ten handbreadths high.
(י) מַקִּיפִין בְּקָנִים, וּבִלְבַד שֶׁלֹּא יְהֵא בֵין קָנֶה לַחֲבֵרוֹ שְׁלֹשָׁה טְפָחִים. בְּשַׁיָּרָא דִבְּרוּ, דִּבְרֵי רַבִּי יְהוּדָה. וַחֲכָמִים אוֹמְרִים, לֹא דִבְּרוּ בְשַׁיָּרָא אֶלָּא בַהֹוֶה. כָּל מְחִצָּה שֶׁאֵינָהּ שֶׁל שְׁתִי וְשֶׁל עֵרֶב, אֵינָהּ מְחִצָּה, דִּבְרֵי רַבִּי יוֹסֵי בַּר רַבִּי יְהוּדָה. וַחֲכָמִים אוֹמְרִים, אֶחָד מִשְּׁנֵי דְבָרִים. אַרְבָּעָה דְבָרִים פָּטְרוּ בַמַּחֲנֶה, מְבִיאִין עֵצִים מִכָּל מָקוֹם, וּפְטוּרִים מֵרְחִיצַת יָדַיִם, וּמִדְּמַאי, וּמִלְּעָרֵב:
(10) Alternatively, one may surround the area with boards that stand upright, provided that there will not be a gap of three handbreadths between one board and the next. When the Sages issued this ruling, they spoke exclusively of a caravan; this is the statement of Rabbi Yehuda, who maintains that a partition of this kind, which consists of only horizontal or vertical elements, is permitted exclusively in exigent circumstances. Otherwise, full-fledged partitions are required. However, the Rabbis say: They spoke of a caravan in the mishna only because they spoke in the present, citing the most typical case. Those traveling in caravans were typically unable to erect full-fledged partitions, so they would surround their camps with ropes or boards. However, the halakha in the mishna applies in all cases. The mishna cites an additional dispute: Any partition that is not constructed of both warp and woof, i.e., vertical and horizontal elements, is not a partition; this is the statement of Rabbi Yosei, son of Rabbi Yehuda. He holds that the vertical boards and the horizontal ropes are not considered a partition, even in the exigent circumstances of a caravan. However, the Rabbis say: One of the two elements, either vertical or horizontal, is sufficient. The Sages exempted a soldier in a military camp in four matters: One may bring wood for kindling from any place with no concern that he is stealing wood from its owners; and one is exempt from ritual washing of the hands before eating; and one is exempt from the separation of tithes from doubtfully tithed produce [demai], i.e., produce purchased from an am ha’aretz, one who is not diligent in separating tithes; and one is exempt from establishing an eiruv.
- You are making a private domain, which needs to be surrounded by walls.
- Walls need to be at least ten tephach (approx 3' high).
- Gaps less than 3 tephach (~9") wide are considered as if they don't exist (think of a chain link or picket fence)
- Gaps less than 10 cubits wide are considered permissible openings, as long as the solid parts outnumber the gaps (in terms of length).
- Bushes and boulders that are >10 tephach and can't be walked through.
(א) אַרְבַע רְשׁוּיוֹת לְשַׁבָּת. רְשׁוּת הַיָּחִיד וּרְשׁוּת הָרַבִּים וְכַרְמְלִית וּמְקוֹם פְּטוֹר. אֵי זוֹ הִיא רְשׁוּת הָרַבִּים מִדְבָּרוֹת וַעֲיָרוֹת וּשְׁוָקִים וּדְרָכִים הַמְפֻלָּשִׁין לָהֶן. וּבִלְבַד שֶׁיִּהְיֶה רֹחַב הַדֶּרֶךְ שֵׁשׁ עֶשְׂרֵה אַמָּה וְלֹא יִהְיֶה עָלָיו תִּקְרָה.
(1) Four domains are distinguishable in connection with the Sabbath: the private domain, the public domain, the semi-public domain, and the exempted region. What is meant by a public domain? Deserts, forests, market places, and thoroughfares opening into them, provided that any such public road is at least sixteen cubits wide and has no roof over it.
In light of the halachic differences between them, it is quite important to determine if an area is a true reshut harabim or merely a karmelit. The precise definitions of these categories have been debated since the time of the earliest Rishonim. The main point of contention is whether an area requires 600,000 people to attain the status of a reshut harabim...
...The Rambam (Hilchot Shabbat 14:1) does not mention that 600,000 people must be present for an area to be considered a reshut harabim. Rashi (Eruvin 6a s.v. Reshut Harabim and Eruvin 59a s.v. Ir), however, writes that a city that does not regularly have 600,000 people is not a reshut harabim, because it has less population than the Jews' encampment in the desert. The practices and the activities of the Jewish encampment in the desert as recorded in the Torah serve as the paradigm for forbidden activities on Shabbat (see Shabbat 73b-74a)...
A major problem with the opinion requiring 600,000 people for a reshut harabim is that the Gemara (Shabbat 6a) describes at length what constitutes a reshut harabim, without any explicit mention of requiring 600,000 people. Surely, the Gemara would not omit such a critical part of defining a reshut harabim. Rav Aharon Lichtenstein has told this author that he believes the opinion of Rashi and the Behag is among the most singularly difficult opinions of Rishonim in all of Halachah!
The Shulchan Aruch (Orach Chaim 345:7) cites (and presumably accepts) the view that an area is a reshut harabim even without 600,000 people, although he does cite the other view as a secondary opinion. The Rama (O.C. 346:3) indicates that he accepts the requirement of 600,000...
...The Mishnah Berurah strongly urges pious individuals (ba'alei nefesh) to be strict and refrain from carrying within an eruv that is based on the lenient opinion. However, he writes that one should not rebuke those who do rely on such eruvin.
- We can rely on Tzurat Hapetach, in lieu of walls in areas that are considered Karmelit.
- Tzurat Hapetach consists of two vertical poles (lechis) under a horizontal "lintel" (Korah)
- The korah has no minimum width, but it should be straight and not sag (how much sagging is considered sagging is a matter of some debate). Aim for no visible sagging. If that is not possible, make sure there is no more than 22 degrees deviation from horizontal, and the wire doeesn't come within 10 tephachim of the ground).
- The Korah should also not move in the wind. Use something like fishing line that can be made taught and straight.
- The Korah should not change directions between lechis. Anywhere you want the wire to turn, you need to place a lechi under the wire.
- The lechis must be directly under the korah, though there can be a gap between them
- The lechis must be at least 10 tephachim high (get poles that are somewhat longer so you can stick them in the ground and still have 3' of pole above the ground).
- There is no minimum width for the lechis, but the wider they are, the easier it is to keep them under the wire, if you're dealing with wires that aren't directly attached to the poles.
- Karpeif (large uninhabited area)--Don't make your eruv so big there is an uninhabited area (i.e., with no tents etc. in it) larger than
- Drawing water

