(Mishnah): If a person's wall was close to his neighbor's wall, he should not build another wall close by unless he distanced it four amos (otherwise the foundations can become damaged) and windows must be above or below a neighbor's facing window or it must be four amos away.

(Gemara) - Explanation #1 - D'Bei R. Yanai: The Mishnah is referring to building a new town.

Explanation #2 (R. Ila): It is referring to a building supported by pillars (even in an old town). The Mishnah requires him to distance one pillar from then next to give room for people to walk there.

Explanation #3 (R. Yosi): It is referring to a wall that is joined to another wall in an 'L' shape.

R. Yosi bei R. Bun: There is a Beraisa that seems to teach that the Mishnah refers to a building wall, as the Beraisa says...

Windows must be above or below a neighbor's facing window or it must be four amos away.

Why above? So that he will not be able to look down into your property.

Why below? So that he cannot stand up and look up at your property.

Why opposite (at a distance of four amos)? So that he does not prevent light from coming into your property.

Question: How far to the side of a window may a person build?

Answer: (Beraisa): One must distance a wall from a window the width of a window.