What’s the difference between an alleyway and a courtyard? TB Eiruvin 12

The default definition of an alleyway (מבוי) is an alleyway that is enclosed by three walls and opens up to the public domain only in one direction (מבוי סתום). Today’s daf TB Eiruvin 12 defines what an alleyway (מבוי) is and what a courtyard (חָצֶר) is.

“Rav Naḥman said: We have a tradition that states: What is the type of alleyway in which carrying is permitted by means of a side post or a cross beam? Any alleyway whose length is greater than its width and has houses and courtyards opening into it. And what is the type of courtyard in which carrying is not permitted by means of a side post or a cross beam, but by an upright board of four handbreadths? Any courtyard that is square.

The Gemara wonders: If it is square, then yes, is it considered a courtyard? If it is round, no, is it not considered a courtyard? The Gemara makes a correction: This is what it is saying: If its length is greater than its width, it is considered an alleyway, and for an alleyway a side post or a cross beam suffices; but if its length is not greater than its width, i.e., it is square, it is considered a courtyard.

The Gemara asks: And by how much must its length exceed its width so that it can be considered an alleyway? Shmuel thought at first to say: It is not considered an alleyway unless its length is double its width, until Rav said to him: My uncle [ḥavivi], Rav Ḥiyya, said this: Even if its length is greater than its width by only a minimal amount, the halakhot of an alleyway apply to it.” (Sefaria.org translation)

Now we know that an alleyway’s length has to be longer than its width i.e. a rectangle and a courtyard can take any shape, for example a square or a circle. Even though both are considered a private domain, when each one opens up to the public domain the adjustment needed to carry within an alleyway and a courtyard is different. All an alleyway needs is a side post, lekhi or a crossbeam, koreh. A courtyard needs to be adjusted by a side board of four handbreadth or two boards of any width on either side of the entrance to the public domain.

Why is the adjustment for the courtyard stricter than the adjustment for an alleyway? Perhaps the difference can be understood in the function each plays. Remember an alleyway needs at least two courtyards entering in to it with at least two houses in each courtyard. The purpose of the alleyway is simply an egress to the public domain. Consequently, one only needs a physical reminder where the private domain ends and the public domain begins. The courtyard functions differently. The courtyard is used for the private activities of the homeowners. They want to maintain this private nature of the courtyard so they need a greater separation of at least a board for handbreadths wide between the courtyard and the public domain.