Rif Eruvin 2b {Eruvin 8b continues ... 11b}

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2b

{Eruvin 8b continues}
all would say that he only places it {horizontally} opposite the short one.

And the halacha is that it is permitted to utilize the space underneath the korah of the alleyway, and between the lechis it is also permitted, like Rava.

And these words are so when it {alleyway} opens into the public domain, but if it enters into a karmelit, no. {Eruvin 9a} For Rav Chanin bar Abba cited Rava: Within the entranceway, he needs another lechi {placed past it} to permit it, whether there is four {handbreadths in this area, a significant space} or there is not four {handbreadths}. And we establish this statement as where it opens to a karmelit, for the species finds its kind and awakens it.

{Eruvin 8b}
And where he sticks two pegs into the walls of the alleyway and places the korah upon them, it does not help at all, for we need a korah on the top of the alleyway, which we lack here.

{Eruvin 9b}
It was stated {by Amoraim}:
If it is visible from within and equal {in thickness and thus not visible} from the outside {of the alleyway = a}, it is judged to be a lechi.
If it is visible from the outside and equal from the inside {=b}, Rabbi Chiyya and Rabbi Shimon beRabbi. One said it is judged to be a lechi and one said it is not judges to be a lechi.

And the halacha is that it is judged to be a lechi.

{Eruvin 10b}
An alleyway which is 20 cubits wide: Abaye said: He makes a strip {pas} 10 handbreadths high and 4 cubits long and erects it across the length of the alleyway in the middle.
{Thus, for 4 cubits, the minimum length of an alleyway, he has now two alleyways at the entrance. The entrance of each into the public domain is not more than 10 cubits wide, and so one may fix the situation by means of a korah or lechi.}

Alternatively, he can do as Rav Yehuda, who said that an alleyway which is 15 cubits wide, he distances {=allows empty space from the wall of} 2 cubits and makes a strip {pas} of 3 cubits {wide=across the entranceway}.
{Now, past the strip is an entranceway of exactly 10 cubits, which poses no problem. The 2:3 seems an omed merubeh issue.}
And where it is 20 cubits wide, he does the aforementioned on this side and that side.
And let us fear lest they abandon the great entrance {of 10 cubits} and enter via the small entrance {of 2 cubits, thus nullifying the fix}?
Rav Yehuda bar Matna said: There is an assumption {chazaka} than a man will not set aside the larger entrance, and enter and leave via the smaller entrance.

{The Mishna had stated:} "But if it have the appearance of a door {tzurat hapetach}, even though it be wider than ten cubits, it need not be made smaller":
{Eruvin 11b}
Rav Chisda said: A tzurat hapetach which is made from the side, he did not accomplish anything.
Rabenu Hai Gaon explains: such that he made it on the side of the wall, such that it is an entrance on a corner {keren zavit}, {Eruvin 10a} and people do not make entrances at corners.

{Eruvin 11b}
And Rav Chisda said: The tzurat hapetach of which they spoke needs to be strong enough to support a door, even if it be a door of straw.

They learnt {tana}: The tzurat hapetach of which they spoke is a board on this side, and board on that side, and a board atop them.