IF THERE WAS FRUIT ON THE TOP OF IT, THE TENANTS ON EITHER SIDE MAY CLIMB UP AND EAT THEM PROVIDED THEY DO NOT CARRY THEM DOWN.
But this is not absolute. A breach in the wall 10 cubits wide connects the two and they can create either one eruv or two - that is, they can be individual spaces, or one combined space.
So far so well. The gemara, though, goes through a number of possible conditions. For example, what if the wall is higher on one side than on the other?
Rabbah son of R. Hunaciting R. Nahman ruled: A wall between two courtyards, one of whose sides was ten handbreadths high and the other one of which was on a level with the ground, is assigned to that courtyard with the floor of which it is level, because the use of it is convenient to the latter but inconvenient to the former, and any place the use of which is convenient to one and inconvenient to another, is to be assigned to the one to whom its use is convenient.
That strikes me as an interesting general principle.
What about a different way to combine two spaces? For example, not a breach in the wall, but a ladders?
Believe it or not, it depends on the ladder!
An Egyptian ladder effects no reduction but a Tyrian ladder does.
What is to be understood by an ‘Egyptian ladder’? — At the school of R. Jannai it was explained: One that has less than four rungs.
The rabbis go on to explain that the Egyptian ladder is light and therefore easily moved while the Tyrian ladder is likely to remain in place throughout the Shabbat. Therefore it is more “permanent” and can be considered like a breach in the wall.
What about two ladders, one on each side?
Abaye ruled: If a wall between two courtyards was ten handbreadths high, and one ladder four handbreadths wide was placed on the one side and another of the same width was placed on the other side, and there is less than a distance of three handbreadths between them, a valid reduction is effected, but if there was a distance of three handbreadths between them, no valid reduction is effected.
That’s a lot of conditions! But wait:
This, however, applies only where the wall was less than four handbreadths thick but if it was four handbreadths thick the reduction is valid even if the ladders were far removed from one another.
Because the wall top is large enough to walk on and becomes a “free space.” A person can climb a ladder on one side, walk along the wall and climb down on the other ladder.
This can now be one space, or two as the owners prefer.