Rif Eruvin 2a {Eruvin 6a continues ... 8b}

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2a

{Eruvin 6a continues}
The Sages learnt {in a brayta}:

How do we create an eruv for alleyways which are open on both sides to the public domain?

{The Tanna Kamma:} We make a tzurat hapetach {=the form of a door, = two posts and a korah atop} on one end {thus transforming it into a "closed" alleyway, open only on one side} and a lechi or korah on the other end.

Chanania says: Bet Shammai say we make a door on one side and a door on the other side, and when he exits or enters, he shuts it. And Bet Hillel say we make a door on one side and a lechi or korah on the other side.

{Eruvin 6b}
It was stated {by Amoraim}:
Rav said: the halacha is like the Tanna Kamma.
And Shmuel said: the halacha is like Chanania, according to Bet Hillel.

And the halacha is like Rav in both of them, as regards the crooked alleyway in what he said that its law was as an open alleyway, and as regards open alleyways in that he holds that the halacha is like the Tanna Kamma. For we establish that the halacha is like Rav in regard to prohibitions.

And these words are that as alleyways which open specifically to the public domain, a tzurat hapetach suffices on one side and a lechi or korah on the other side. But the public domain itself, we cannot make an eruv except via doors on this side and that, and this only where they are shut at night, for Rabba bar bar Channa cited Rabbi Yochanan: Yerushalayim, if not for its doors being shut at night, we would be liable for it as a public domain.

And that crooked alleyway which was in Nehardea, and they cast upon it the stringencies of Rav and Shmuel and required doors, this is no difficulty, for Nehardea was the place of Shmuel.

{Eruvin 7b}
Rav said: An alleyway which is breached in its fullness to a courtyard, and the courtyard is breached opposite it to the public domain, the courtyard is permitted and the alleyway is forbidden.

{Eruvin 8a}
The courtyard is permitted, even though the public intrudes upon it, for we learnt in a brayta: A courtyard that the public enters here and leaves there is a private domain as regards Shabbat and a public domain as regards ritual impurity.

And the alleyway is forbidden, for it is an alleyway which is "open" to the public domain.

{Eruvin 8b}
It was stated {by Amoraim}:
An alleyway constructed in the shape of a nadal {=centipede}: To explain nadal, it is an insect which has many feet to each side, and it is "open" {to the public domain}.

{To explain the image, each line represents an alleyway. The alleyway is open to the public domain at each spot marked by an X.}

Abaye said: He makes a tzurat hapetach for the large one {on one side of the horizontal line} and all the rest will be permitted by means of a lechi or a korah.

{Abaye holds here like Shmuel that a crooked alleyway it has the status of a closed alleyway. By placing a tzurat hapetach on one side and a korah on the other, the horizontal alleyway is covered. The other alleyways are all closed by means of the crookedness, such that each only needs either a lechi or korah.}

Rava said to him: Like whom? Like Shmuel who said that it {the crooked alleyway} is judged like a closed alleyway?! But there was a crooked alleyway in Nehardea and they took into consideration Rav's opinion!
{And so here, one could trace a line from the korah at the end of each vertical alleyway to the korah at the one end of the horizontal, or even to the korah at the end of another vertical!}
Rather, Rava said: he makes a tzurat hapetach for each of them on one side, and the other side may be permitted via a lechi or korah.
{Since there is only one lechi or korah in the entire production, one cannot trace a line from the korah towards another korah, but will always hit a tzurat hapetach on the other side.}

From this we derive that the halacha is like Rav who said that the halacha is like the Tanna Kamma {in making an eruv for an open alleyway}, for Rava holds like Rav, and we have established that in every dispute between Abaye and Rava, the halacha is like Rava except for יע"ל קג"ם.

It was stated in the name of Rav Kahana the Great: An alleyway whose one side is long and the other side is short: If {the difference between them is} less than 4 cubits, he places the korah at the diagonal; 4 cubits, he only places the korah {horizontally across the alleyway} by the short wall.

To explain, if the long one is more than the short one by 4 cubits, it is reckoned an alleyway unto itself and needs a partition {wall} opposite it, and if the long one is more than the short one by less than 4 cubits, it is not like its own alleyway, and therefore he places the korah upon it diagonally.

Rava said: In both cases, he only places the korah {horizontally} next to the short one.
What is the reason? The korah is because of heker {to let people know the transition of Shabbat domain} and diagonally, it is not a heker.

And if there is in {the length of} the diagonal more than 10 cubits,