Ohalot, Chapter Thirteen, Mishnah Two
1) [With regard to] a window made for letting in air, its minimum size is that of a hole made by the drill.
2) If a house was built outside it, its minimum size becomes one handbreadth square.
3) If a roof was placed at the height of the middle of the window, the minimum size of the lower part is one handbreadth square and of the upper part that of a hole made by the drill.
Section one: If a window was made for letting in air, then it must have been made on the outside of a building. It will also let in light. Therefore, it allows impurity in and out according to the smaller measure, that of a hole made by a drill (see yesterday’s mishnah). If the hole was between two rooms, it would need to be at least a handbreadth in order to allow impurity through.
Section two: When the window was in the outermost wall of the house, it needed to be only the size of the drill-hole. But now that someone builds a house next to it, it needs to be larger, a full square handbreadth, to allow impurity to travel through.
Section three: In this case, a roof was built right next to the window, assumedly because the city is on a hill. Now the top of the window faces outside and the bottom of the window opens into the adjacent building. The mishnah looks at this as two different windows. The top part needs to be only as large as the drill but the bottom part must be a handbreadth.