Chapter 7, Mishna 2
When two courtyards are separated by a significant wall, they lose the option to make a joint eruv that permits carrying objects between them on Shabbos. What is a significant wall? Can we use the top of such a wall for storage? How can we diminish such a wall, so that the joint eruv becomes possible again? Can ladders on either side create a passageway between the courtyards? What about a step-stool or an overturned basin?