Eruvin Chapter 2, Mishnah 3. This mishnah deals with two separate areas - a courtyard and a balcony - that did not make an eruv with one another, and with an object standing at a certain height in one of them. The question under discussion is: in whose domain does that object stand?
The question of domain has a practical consequence: if one of the areas has exclusive control of the object, its residents may carry items out of their homes and place them on it. But if both areas control it, both are forbidden to do so, since a place that is conveniently usable by two domains is treated as an area under joint control, and in order to use it both must make an eruv together. Since they did not make an eruv, that area is forbidden to both of them.
The picture before us:
The mishnah opens: "Anshei chatzer v'anshei mirpeses sheshachechu v'lo eirvu" - the people of a courtyard and the people of a balcony who forgot and did not make an eruv. The case is a building in which rooms open onto the courtyard, and above them there is a balcony (gezuztra) with homes opening onto it, and a flight of stairs descending from it into the courtyard. The people of the courtyard are those living in the homes that open onto the courtyard, and the people of the balcony are those living in the homes that open onto the balcony. The two forgot and did not make an eruv with one another, but each group made an eruv for itself: the people of the courtyard carry from their homes into the courtyard, and the people of the balcony carry from their homes onto the balcony, but they do not carry from one domain into the other, since they did not make an eruv chatzeiros together.
"Kol shegavoah asarah tefachim - lamirpeses":
An object standing in the courtyard that is ten tefachim high, when it is adjacent to the balcony, is in the exclusive domain of the balcony. The reason: something ten tefachim high is not convenient for the people of the courtyard to use, whereas for the people of the balcony - since it is within ten tefachim of the level of the balcony - it is convenient to use. Therefore the people of the balcony may bring items to it from their homes, and the people of the courtyard may not.
"Pachos mikan - lechatzer":
If the object is lower than ten tefachim, it belongs to the courtyard. The Gemara qualifies the wording "to the courtyard": it does not mean to the courtyard alone, but rather "af lechatzer" - also to the courtyard, that is, to the courtyard in addition to the balcony. And since it is in the domain of both, both are forbidden to use it and to place objects on it.
"Chulyas habor v'hasela":
The mishnah continues and lists additional types of objects standing in the courtyard: 'chulyas habor' - the earth dug out of the pit or the well, which is left as a kind of raised rim around the well; and 'hasela' - a rock, in general. With these too:
Ten tefachim high - they belong to the balcony, in its exclusive domain, and the people of the balcony may place objects on them.
Lower than that - they belong to the courtyard, meaning also to the courtyard, and therefore both are forbidden to use them.
"Bameh devarim amurim - bismuchah":
When does the rule apply that something ten tefachim high is in the domain of the balcony alone? Only "bismuchah" - when that object is adjacent to the balcony. But "bemuflag" - when it is distant from it horizontally, not in height - then even if it is ten tefachim high or even higher, it belongs to the courtyard, and here too this means also to the people of the courtyard, and therefore both are forbidden to place their objects on it until they make an eruv.
"V'eizo hi semuchah":
What is the definition that distinguishes between adjacent and distant? "Kol she'einah rechokah arbaah tefachim" - anything that is not distant by four tefachim, meaning that it is within four tefachim of the balcony. This closing clause qualifies the opening of the mishnah: it is not enough for the object to be ten tefachim high, it must also be within four tefachim of the balcony. Otherwise it belongs to the courtyard and to the balcony together, and both are forbidden to use it until they make an eruv together.
In summary: in this mishnah we learned about a courtyard and a balcony that did not make an eruv with one another, and each made an eruv for itself. An object standing in the courtyard that is ten tefachim high and adjacent to the balcony, within four tefachim, is in the exclusive domain of the balcony, and its residents may place items on it from their homes. An object lower than ten tefachim, and likewise a high object that is set apart and distant four tefachim from the balcony, belongs also to the courtyard, and since control of it is shared, both are forbidden to use it until they make an eruv together. The same law applies to the rim of a pit and to a rock.