Gemara

58b (Mishnah): One does not get a Chazakah for a Mitzri window. One gets a Chazakah for a Tzuri window. A Mitzri window is one which a person's head does not fit.

R. Yehudah says, if it has a frame, even a small window gets a Chazakah.

59a (R. Zeira): If Reuven's window is less than four Amos above the ground, he gets a Chazakah. The neighbor can protest (before Reuven gets a Chazakah). If it is more than four Amos above the ground, he does not get a Chazakah, and the neighbor cannot protest.

(R. Ila'i): Even if it is higher than four Amos, he does not get a Chazakah and the neighbor can protest.

Suggestion: They argue about whether or not Kofin Al Midas Sedom (we force one to let another person benefit, if he himself will not lose). R. Zeira holds that we force, and R. Ila'i holds that we do not.

Rejection: No, all agree that Kofin Al Midas Sedom. Here is different, for sometimes Reuven stands on a bench and sees through the window.

David came in front of R. Ami (to request to build a window above four Amos, against his neighbor's protest). R. Ami sent him to R. Aba bar Mamal, and instructed R. Aba to forbid him, like R. Ila'i.

(Shmuel): A window made for light (surely, it is meant to be permanent) gets a Chazakah regardless of its size.

59b (Mishnah): Reuven may not open a window into a jointly owned Chatzer.

(Beraisa): A case occurred in which Reuven opened a window into a joint Chatzer. R. Yishmael b'Rebbi Yosi said that he has a Chazakah. R. Chiya ordered him to close the window.

(Rav Nachman): If Shimon built a wall blocking light from entering Levi's window, Shimon is immediately Muchzak. If Levi was truly Muchzak in his window, he would never allow someone to block it.

Rishonim

Rif and Rosh (3:73,75): The Halachah follows R. Ila'i and R. Chiya.

Rosh: The Rashbam explains that R. Zeira and R. Ila'i discuss a Tzuri window. The Gemara connotes like this. The Mishnah taught that a Tzuri window has a Chazakah, and they qualify when this is true. The Gemara did not say 'we learned only...' because this would imply that R. Zeira's primary Chidush is that above four Amos, he has no Chazakah. This is not true. His primary Chidush is that above four Amos, one cannot protest. R. Ila'i argues about this. R. Tam says that they discuss a Mitzri window. A Tzuri window is fixed, and even above four Amos it he has a Chazakah, even if it is not for light. Even though one cannot fit his head in a Mitzri window, when he passes by he sees what his neighbor does in the Chatzer, and there is Hezek Re'iyah (loss of privacy). R. Tam brought a proof from the Mishnah that says that a window above four Amos has a Chazakah. This is no proof. It discusses a window in an Aliyah (upper story), or when a document says that the window is for light. Also R. Yosef says that they discuss a Mitzri window. R. Yonah proves this, for right after this, the Gemara brings Shmuel's law that a window made for light, of any size, gets a Chazakah. If R. Zeira discussed a Mitzri window, Shmuel's law, which discusses a Mitzri window, should have been brought beforehand! Also, before R. Zeira, the Gemara discusses why the Mishnah defines a Mitzri window. If R. Zeira discussed a Mitzri window, the Gemara should have cited this from the Mishnah before R. Zeira's Halachah!

Rambam (Hilchos Shechenim 7:5): If Reuven wants to open a window to Shimon's Chatzer, be it big or small, above or below, Shimon can stop him. He can say 'you will harm me through seeing. Even if it is high, you will climb on a ladder and look.'

Rambam (6): If Reuven opened a window to Shimon's Chatzer and Shimon pardoned this, Reuven has a Chazakah for the window. Shimon cannot later demand that he close it. What is the law of the window he wants to open? If a person's head fits inside, or it was less than four Amos (above the ground) even if one's head does not fit inside, Shimon may not build opposite it or to the side, unless he distances four Amos.

Rebuttal (Ra'avad): I do not agree to the Rambam or his Rebbi about this. The Gemara distinguished between a Chazakah above and below four Amos only for a Tzuri window. There is never a Chazakah for a Mitzri window! All my Rebbeyim agreed.

Rambam (7): If one's head does not fit inside and it is more than four Amos (above the ground), Shimon may build opposite it or to the side. He can say 'I allowed you to build it only because it is small and high. I did not grant you a Chazakah to forbid me to build near it.' This is when he opened it for use or air. If he opened it for light even if it is very small and high, since Shimon did not protest, Reuven has a Chazakah. Shimon may not build opposite it or to the side, unless he distances four Amos, lest he dark Reuven's house. He pardoned to him regarding light. Similarly, if Levi had a Chazakah for a window, and David built opposite it or to the side, or sealed the window, and Levi was quiet, he cannot later demand to open the window or distance the building. Since he was silent, he pardoned. If one's light is removed, he is not silent unless he pardoned.

Poskim

Shulchan Aruch (CM 154:6): If Reuven wants to open a window to Shimon's Chatzer, be it big or small, above or below, Shimon can stop him. He can say 'you will harm me through seeing. Even if it is high, you will climb on a ladder and look.'

Beis Yosef (DH Mi): R. Yonah and the Rosh agree with R. Tam. It seems that the Rambam agrees. The Nimukei Yosef says that R. Tam says that the Rashbam later retracted. The Rashba says that the custom is to rely on R. Tam.

Beis Yosef (DH ul'Divrei): R. Zeira and R. Ila'i argue about a small window below four Amos. Even though there is no Hezek Re'iyah, he can say 'I do not want you to get a Chazakah against me.'

Shulchan Aruch (10): If he wants to open a window such that a person's head fits inside, or it was lower than four Amos even if one's head does not fit inside, Shimon may not build opposite it or to the side, unless he distances four Amos, like we will explain (Sa'if 21). If one's head does not fit inside and it is higher than four Amos, Shimon may build opposite it or to the side. He can say 'I allowed you to build it only because it is small and high. I did not grant you a Chazakah to forbid me to build near it.' This is when he opened it for use or air. If he opened it for light, even if it is very small and high, since Shimon did not protest, Reuven has a Chazakah. Shimon may not build opposite it or to the side, unless he distances four Amos, lest he make it dark for Reuven. He pardoned to him regarding light.

Beis Yosef (DH v'Chosav Nimukei): The Nimukei Yosef says that it has a Chazakah only if it has Tzuras ha'Pesach (sticks on both sides, and one on top) or a frame. If not, it is like an opening made by mice, and it has no Chazakah, even for light. I say that he learned this from a Tosefta in which R. Meir requires this. Our Stam Mishnah is R. Meir. Perhaps the Rif, Rambam and Rosh did not write this because a Stam window has Tzuras ha'Pesach or a frame.

Bedek ha'Bayis: R. Tam says that if it is for light, there is no Shi'ur, even if it is less than a Tefach. It seems that the Rambam agrees. This connotes that it does not need Tzuras ha'Pesach or a frame, for normally one does not make them for a window smaller than a Tefach.

Beis Yosef (DH v'Chen): The Rashbam explains that since it is a constant matter, for one needs light every day, Shimon should have protested. Since he did not, we can say that Reuven had permission. The Nimukei Yosef connotes that it must have Tzuras ha'Pesach or a frame. The Rambam disagrees.

Beis Yosef (DH v'Yesh and DH v'Zeh): The Ba'al ha'Itur (cited in Teshuvos Maimoniyos Mishpatim 71) proves from the Yerushalmi (which equates this law to Tum'ah) that it must be at least a Tefach. The Ramban learns from the Yerushalmi that the Shi'ur is the same as for Tum'ah, i.e. the width of a drill, which equals the width of the hole in a plow.

Rema: The judge uses his judgment to determine if it was made for light. It depends on the kind of window.

Beis Yosef (DH v'Zeh): The Ramban says that a window made for light is wider on the inside than the outside (also Teshuvos Maimoniyos above says so), or the house needs light. The Rambam connotes that it depends on the intent of the one who made the window.

Beis Yosef (DH Kosav, citing the Rashba (3:182)): A window made for light is one that illuminates a dark place. The custom is that one may stop a neighbor from building near any window that light comes through, even if there are other such windows. It depends on what Beis Din decides.