81:2 A private domain is an area that is at least four handbreadths by four handbreadths, which is a space large enough to be used in some capacity, surrounded by partitions at least ten handbreadths high, even if they are not completely solid. (A handbreadth is about 3 or 3.5 inches.) A trench ten handbreadths deep and four handbreadths square also counts as a partition for this purpose. A pit that is ten handbreadths deep and four handbreadths by four handbreadths wide, a mound that is ten handbreadths tall and four by four wide, a utensil like a chest that is ten handbreadths tall, and a barrel whose circumference is the equivalent of four handbreadths by four handbreadths - all these are considered their own, individual private domains. This is true even if they are standing in a public domain or a carmelis. The airspace above a private domain is considered a private domain all the way up to the sky. The tops of the partitions surrounding a private domain are also considered a private domain. If there are recesses in the partition that open towards the private domain, even if they are hollow from one side to the other, they are subordinate and considered part of the private domain because they can be used from inside the private domain.
81:3 A public domain is made of streets and market places that are sixteen cubits by sixteen cubits, which was the width of the road from the Levites' camp in the wilderness. (16 cubits is about 24 feet.) Also, the roads that one used to travel from city to city were 16 cubits wide. Anything in the public domain that is not more than three handbreadths above the ground, even thorns or dung that people don’t walk on, is considered subordinate to the ground and therefore a part of the public domain. A hole in a public domain that is not deeper than three handbreadths is considered part of the public domain. The recesses in walls that face the public domain to the exclusion of the private domain are considered subordinate - and therefore a part of - the public domain so long as they are lower than three handbreadths from the ground. If they are higher than three handbreadths, we must take their overall dimensions into account. If they are four handbreadths by four handbreadths but lower than ten handbreadths, they are considered a carmelis; higher than ten handbreadths, they are considered a private domain. If it's not four by four, it is considered a makom patur (exempt area) whether it is above or below ten handbreadths in height. Alleys that open into the public domain are sometimes considered public domains themselves, and other times they are considered a carmelis; there are many factors and laws regarding this matter. Some authorities say that if there aren't 600,000 people traversing an area daily - based upon the population of the camps in the wilderness - then that area isn't a public domain but a carmelis. This would mean that nowadays we have relatively few real public domains. Nevertheless, a G-d-fearing person should be strict on himself.