Yerushalmi Eruvin 7:2 ירושלמי עירובין ז׳:ב׳

MISHNAH: If a wall between two courtyards is ten [hand-breadths] high and four wide, one makes two eruvin and cannot make one eruv²A wall 10 hand-breadths high is a partition which separates the courtyards completely. In addition, an opening less than 4-by-4 in size is difficult to use; it cannot substitute for a door.. If produce was lying on top of it, both partiess may climb up and eat them; only they may not take them down³If on the top of the wall there is any food, it may be taken from either side on the Sabbath if it is accessible by ladders before the start of the Sabbath. If the top of the wall is 4 hand-breadths wide it defines a domain by itself which cannot be included in the eruv of either side. In this case the food may not be taken down.. If a wall was torn down up to ten cubits, one makes two eruvin but if they wish they make one eruv. More than this, one makes one eruv but may not make two⁴As mentioned in Note 1, any wall not higher than 10 hand-breadths may be (but need not be) disregarded..

HALAKHAH: Paragraph. ⁴⁵Mishnah 2.“If a wall between two courtyards,” etc. Why does it have to be four [hand-breadths] wide? Even if it is not four [hand-breadths] wide? Because of what is stated later: “both sides may climb up and eat them.³If on the top of the wall there is any food, it may be taken from either side on the Sabbath if it is accessible by ladders before the start of the Sabbath. If the top of the wall is 4 hand-breadths wide it defines a domain by itself which cannot be included in the eruv of either side. In this case the food may not be taken down.” If it is not four [hand-breadths] wide, Rebbi Abba in the name of Rav: it is forbidden for both sides. Rebbi Ze`ira in the name of Rav: it is permitted for both sides. The argument of Rebbi Abba seems inverted⁴⁶Since there seems to be no reason to forbid taking the food in this case.. There Rebbi Abba said, Rav Jehudah in the name of Samuel: If he threw it and it came to rest on top of a partition he is liable⁴⁷If somebody stands in the public domain and throws something for more than 4 cubits and it come to rest on the narrow top of a fence of a private domain it is considered as coming to rest inside the private domain; the thrower is liable for a biblical Sabbath violation.. And here, he says so⁴⁸Since he forbids access to the top of a narrow wall between private domains he must hold that this is a separate domain; then there is no difference whether the wall be four hand-breadths wide or not, against the statement of the Mishnah.
The answer is that if one considers the private domain as filled up with dust or pebbles to the height of the fence, the top of the fence clearly belongs to the private domain. But here one is speaking of a wall between two private domains where the argument is inappropriate.? There one considers it as if filled with dust and pebbles. Here, what do you have? You should know this since Rebbi Aḥa, Rebbi Ḥinena said in the name of Rebbi Joḥanan: Ledges and walls high ten [hand-breadths] and wide four are permitted for both sides, on condition that he may not exchange⁴⁹Šabbat Chapter 1 Note 170, Babli Šabbat 8b, 99b.. Rebbi Joḥanan⁵⁰For reasons of chronology it is difficult to accept the reading “R. Joḥanan”. Pene Moshe emends to “R. Ḥanan”. asked. The argument of Rebbi Abba seems inverted. There Rebbi Abba said, Rav Jehudah in the name of Samuel: If he threw it and it came to rest on top of a partition he is liable. And here, he says so? You are saying there, one considers it as if filled with dust and pebbles; here it is the same.

We heard about partitions in a private domain, we heard about partitions in a public domain⁵¹The statement about partitions in private domains is Mishnah 2, about public domains is the one of Rav Jehudah quoted in the preceding paragraph.. ⁵²Cf. Chapter 6 Note 178. Here one is forced to read אֵין. It is clear from the earlier discussion (Note 37) that two connecting ladders on a wall permit but do not require making a single eruv (Babli 60a). Rebbi Abba, Rav Jehudah in the name of Samuel: If a wall is surrounded by ladders on both sides, one makes two eruvin and may not make one eruv. Rebbi Yose ben Rebbi Bun said, since they are not at the same place, are they like doors?

It was stated: The raising of a ramp is one cubit when he progresses three except for the ramp of the altar which collects ten hand-breadths for three and one third fingers, since the altar was ten cubits [high] and its ramp thirty-two⁵³Cf. Babli Zevaḥim 63a. The standard for building ramps was height: base = 1:3 except the altar where it was 10:32. The sentence “which collects ten hand-breadths for three and one third fingers” is corrupt. In Arukh, s. v. כבש the reading is וְכָל־תֵּל הַמִּתְלַקֵּט עֲשָׂרָה טְפָחִים מִתּוֹךְ חָמֵשׁ אַמּוֹת הוּא שְׁלִישׁ “any hill which rises 10 hand-breadths for 5 cubits (30 hand-breadths) is one third.”.

If one made a bench along the entire wall which is four hand-breadths wide one makes two eruvin and may not make one; with four cubits one makes one eruv and may not make two⁵⁴One might conjecture that for “and may not make one” one should read “and if they wish they may make one” since if the wall was not higher than 10 hand-breadths over the step which is 4 wide the Mishnah allows making one eruv. The new statement of the baraita is that if the step was easy to use, being 4 cubits wide, the wall less than 10 hand-breadths high becomes a permanently open door and one eruv is mandatory..

That is if it is large but if it is small by most of it. What is large and what is small? Any where most of it is at least ten, by ten; therefore less than ten by most of it. That is meant in the middle, but not from the side. What is on the side and what is in the middle? If what is standing is four [hand-breadths] it is in the middle, otherwise it is on the side⁵⁵This is a direct commentary on the last statement of Mishnah 2 that if the wall is torn down up to 10 cubits one may make one eruv. If the entire wall is only 11 cubits and 10 are missing there is no wall and it should be impossible to make 2 eruvin. Therefore the statement of the Mishnah may apply directly only to walls more than 20 cubits long..