Gud Achit, Gud Asik - Considering walls as though they extended upward or downward to a point in space

LinksGeneral Rules Considering this principle a Law told to Moses at Sinai: Eruvin 4b; Succah 6b
Whether one may use this principle of the top of the wall is a space over which people travel [Mechitzah haNidreset]: Eruvin 89b

Applications to Laws of Shabbat