Eruvin 93B

גידוד חמשה ומחיצה חמשה ⁠— אין מצטרפין עד שיהא או כולו בגידוד או כולו במחיצה.

An embankment, a height disparity between two surfaces of 5 handbreadths, and an additional partition of 5 handbreadths do not join together to form a partition of 10 handbreadths, the minimum height for a partition to enclose a private domain. It is regarded as a partition of 10 handbreadths only if the barrier is composed entirely of the embankment or if it is composed entirely of a partition.

מיתיבי: שתי חצירות זו למעלה מזו ועליונה גבוהה מן התחתונה עשרה טפחים, או שיש בה גידוד חמשה ומחיצה חמשה ⁠— מערבין שנים, ואין מערבין אחד. פחות מכאן מערבין אחד, ואין מערבין שנים!

The Talmud raises an objection from a baraita: If there were two courtyards, one above the other, and the upper one was 10 handbreadths higher than the lower one, or if it had an embankment of 5 handbreadths and a partition of 5 handbreadths, the two courtyards are considered separate domains and they establish two eiruvin, one for each courtyard, and they do not establish one eiruv. If the height disparity was less than 10 handbreadths, the two areas are considered a single domain, and they establish one eiruv and they do not establish two eiruvin.

אמר (רב): מודה רב חסדא בתחתונה, הואיל ורואה פני עשרה. אי הכי: תחתונה ⁠— תערב שנים ולא תערב אחד, עליונה ⁠— לא תערב לא אחד ולא שנים.

Rav said: Rav Ḥisda concedes that an embankment and a partition combine with regard to the lower courtyard, since it faces a wall of 10, i.e., there is a full partition of 10 handbreadths before its residents. The Talmud raises a difficulty: If so, according to this reasoning, the residents of the lower courtyard, from whose perspective there is a valid partition, should establish two eiruvin, i.e., an independent eiruv, and do not establish one eiruv together with the upper courtyard, while the residents of the upper courtyard establish neither one eiruv nor two. The residents of the upper courtyard neither establish an eiruv on their own, as it is breached into the lower one, nor can they establish an eiruv together with the lower courtyard, because the latter is separated from it.