Eiruvin 1:9-10

Eiruvin 1:9

The members of the caravan (in mishna 1:8) may surround their camp with three ropes, one above the other, so long as the gap between one rope and the next is less than three handbreadths (about nine inches). The width of the three ropes combined will be more than a handbreadth, so the entire height from the ground to the top of the highest rope will be ten handbreadths (about 30 inches, the minimum height of a valid partition).

Eiruvin 1:10

The caravan can also surround their camp with standing reeds (at least ten handbreadths tall) so long as there are not more than three handbreadths between one reed and the next. Rabbi Yehudah says that the partition of ropes (only horizontal portions) or of reeds (only vertical portions) only work in the case of a camping caravan but the Sages say that these partitions work in any case, the caravan is just an example of a situation where such a solution might typically be employed. Rabbi Yosi bar Rabbi Yehuda says that even in the case of a caravan, a partition is invalid if it doesn’t include both horizontal and vertical portions; the Sages say either horizontal or vertical portions. The Sages also permitted four things in the case of an army camp: they may acquire wood even from places normally prohibited, they need not ritually wash their hands before eating, they need not tithe demai (doubtfully-tithed produce), and they need not make an eiruv to permit carrying on Shabbos.