Rif Succah 2a {Succah 4b; 6b - 7a}

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2a

{Succah 4b}
"And if it does not have 3 walls":

{Succah 6b}
The Sages learnt {in a brayta}: Two according to their law {whatever the halachic requirement is for the length of a wall} and the third even a handbreadth. Rabbi Shimon said: Three according to their law and the fourth even a handbreadth.

And we establish like the Sages.

{Succah 7a}
And that handbreadth, how shall he place it?
Rabbi Simon cited Rabbi Yehoshua ben Levi: He makes a loose {thus large} handbreadth and places it less than 3 handbreadths close to the wall. {Thus, lavud will be in effect, halachicaly closing the area, and we will have a wall of 4 handbreadths.}

Rav Yehuda said: A succah which is constructed like an alleyway is valid.