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1. Babylonian palm trees placed next to a wall lessen the height of the wall (regarding Eruvin).
2. There is a dispute about whether a tree that is used by the residents of two courtyards to climb from one courtyard to the other allows them to make a single Eruv together.
3. A ditch that is ten Tefachim deep between two courtyards prevents the residents from making a single Eruv.
4. The Mishnah mentions ways to effectively bridge the gap between the two courtyards.
5. Straw separates two courtyards when it is piled up like a wall, but it does not necessarily serve to fill a gap.
A BIT MORE
1. Because of their heavy weight, they are considered permanently placed there.
2. Some say that it does. Even though they are not permitted to use the tree on Shabbos, it still serves to connect the two courtyards. Others say that the tree cannot make the two courtyards be considered like one regarding Shabbos, since the tree may not be used on Shabbos, and thus the laws of Shabbos prohibit them from becoming one courtyard.
3. The ditch prevents them from making a single Eruv even when it is full and does not separate the two courtyards from each other, if it is full of items that people use (and will remove from the ditch, thereby restoring the physical separation between the two courtyards), such as straw. However, if the ditch has been filled with earth or rocks, the courtyards are considered a single Chatzer.
4. If a plank that is four Tefachim wide is placed from one end of the ditch to the other, or if a bridge is created from a house in one yard to a house in the other, the residents may choose to treat the two courtyards as one Chatzer or as two separate ones.
5. The Gemara points out a contradiction. A wall of straw between two courtyards separates the courtyards from each other (so that two Eruvin must be made). However, the Mishnah (see #3) says that straw does not fill a ditch between two yards unless it is supposed to remain there. The Gemara answers that a wall serves as a separation as long as it is physically present, but straw in a ditch is only deemed a filler when one has no intention to remove it.
Next Daf Index to Revach for Maseches Eruvin