If one opens a window lower than 4 amot, and the neighbors don't protest, then the one who did so can keep it, as a chazakah. A high window, however, doesn't get the status of chazakah, because it's not the kind of thing that people protested - except that R. Ilai disagrees. Which some say means not permitting one to protest for the sake of not being too selfish, like the traits of the biblical Sodom. Also, a new mishnah! On the airspace of a shared courtyard - for example, a beam that reaches into the courtyard, which will establish ownership if it's not protested. Plus, another mishnah, with more on what is permissible in a home on a shared courtyard -- for example, opening a loft into a home, but not into the courtyard.