Rif Eruvin 1a {Eruvin 2a ... 5a}

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1a

{Eruvin 2a}
MISHNA:
If an alleyway {mavuy} be higher than twenty cubits, it should be made smaller {=lowered}.
Rabbi Yehudah said: "This is not necessary."

If it be wider than ten cubits, it should be made smaller {=narrower}, 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 {=narrower}.

Gemara:
How should he make it {the entranceway to the alleyway} smaller {=lower}?
He should put upon {and across} it a korah {=board} from twenty cubits and below, [such that the hollow space should be twenty cubits {or less}]¹.

For² we say {Eruvin 3a}: If a portion of the korah was within twenty {cubits} and a portion was higher than twenty; if a portion of the sechach {sukkah covering} was within twenty and a portion was higher than twenty: Rabba said: for an alleyway it is valid and for a sukkah it is invalid. And we conclude: Rava said: Both this and that are valid. The hollow of a sukkah we learnt {in the Mishna}; the hollow of an alleyway we learnt {in the Mishna} - that it must be twenty cubits.

{Eruvin 4b}
If it was higher than twenty cubits and he comes to reduce it{s height by placing a korah on the floor to raise the floor}, he may reduce it with a korah whose width is only a single handbreadth, and it is sufficient,

{Eruvin 5a}
for a korah functions as a heker³ and the heker from below is like the heker above {at the top of the alleyway}. And just as the one above is sufficient when a handbreadth {wide}, as we learn {tnan} -- It is sufficient for a korah to be a handbreadth wide -- so too the korah he uses to reduce below {by adding it to the floor} [it]⁴, it is sufficient that it is a handbreadth wide.

And if it {the korah, or the hollow} is less than ten handbreadths, he should carve out into the floor an area of 4 cubits X 4 cubits⁵, in order to complete it to be {a height of} ten handbreadths, like Abaye. And even though Rav Yosef, who is his {Abaye's} teacher, argues on him, says that he may carve into it {the floor} an area of 4 {handbreadths} x 4 {handbreadths}, since our sugya is like Abaye in that the {minimum} span {meshech} of an alleyway is 4 cubits, the halacha is like him⁶.

It was stated {by Amoraim}: An alleyway which was broken down by its side by its head:

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{The dot showing where the wall was broken down: at the corner of the alleyway, at its head}

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Footnotes:
1: This text was placed in brackets on the suggestion of Bach - was not in old manuscripts of Rif. Without the text, all it means is that one should place a korah across the entranceway of the alleyway, and the board should not be higher off the ground than twenty cubits. This is also a valid interpretation if we introduce the text in brackets.

However, we seem to be dealing with an alleyway which previously had a korah across, except that the board was higher than twenty cubits, and we now wish to "lower it". The suggestion put forth in the text is problematic. Is he suggesting that one place another board lower, within twenty cubits? This is difficult, for we know from elsewhere that the korah must be placed across the top of the alleyway, and not resting on pegs stuck into the alleyway's walls. Perhaps resting on its own posts? Or perhaps the text is inexact, and we are discussing putting the first korah up. Or else, we are reducing the height of the alleyway from an infinite height, bound only by the clouds, to one bounded by a korah on top.

Another interpretation of the suggestion, which is in fact seems fleshed out later in the text of the Rif itself, and which I adopt, is that this second korah is being placed near the floor, thus raising the floor. This would then be the meaning of the explanatory bracketed text. By raising the floor with this threshold korah, the hollow between the new floor-korah and the original korah is reduced, such that the hollow is less than twenty cubits.

For a better explanation of the need for the word "hollow," see next footnote.

2: Here is a justification by the Rif of why he said specifically that the "hollow" must equal 20 cubits rather than that the korah must be twenty cubits off the ground. For the korah need not be twenty cubits off the ground. We are concerned with the intervening space, and the statement of Rabba is proof for this.

3: to notify people of its presence and thus the existence of the alleyway as distinct from other domains

4: removed by a gloss

5: 4 cubits X 4 cubits would be a respectable, halachically relevant space, such that it has validity. Here, when lowering the floor, we may not lower it is only one place, but must lower it for a respectable area. In this, it is distinct from the bottom korah used to raise the floor, in that the korah need but span across the floow, with a thickness of a handbreadth, since it operated on the level of heker. Carving into the floor requires that this adaptation of the hollow take place over a wider area.

Note that Bach emends and crosses out "by four cubits", such than it is specifying how far along by the entrance to the alleyway the floor must be carved. It must by for a length of 4 cubits in, for 4 cubits is the minimum length of an alleyway, as we shall see shortly. Thus, we must have a halachically minimum "alleyway" (of 4 cubits) at the entrace to our alleyway whose height from floor to korah is the minimum 10 handbreadths.

6: As this din is based on the minimum span of an alleyway being 4 cubits.