907. Starting Hilchos Eiruvin

Eiruvin 1:1

Under Biblical law, when several neighbors in private homes share a common courtyard, they may all carry throughout the courtyard, and from their homes to the courtyard and vice versa, because the courtyard is a private domain in which carrying is permitted. Similarly, if an alley has a pole or a beam at its entrance, all the residents of the alley may carry throughout it, as well as from their courtyards to the alley and vice versa because the alley is a private domain. Similarly, a city that is surrounded by a wall of at least ten handbreadths in height (about 30”) and whose gates are locked at night is a private domain. Again, this is all under Biblical law.

Eiruvin 1:2

By rabbinic enactment, neighbors are not permitted to carry in a private domain that is divided into different dwelling units unless they all join together in an “eiruv” before Shabbos begins. This rule was instituted by King Solomon and his court, and it applies to courtyards, alleys and cities.