Eiruvin 5:1-2

Eiruvin 5:1

The boundaries of towns are extended as follows: let’s say that the structures on the edge of the town are not in a straight line. Rather, one house extends further than another, or one tower extends further than another, or there are ruins at least ten handbreadths high (about 30”), or bridges or mausoleums that have apartments for the caretakers. In all of these cases, measurements are made from such structures and the boundary is drawn in the shape of a square, so that they gain the benefit of the corners (because the diagonals are longer than if the boundary was drawn in the shape of a circle).

Eiruvin 5:2

A karpeif is a space of 70 2/3 cubits (about 106 feet). Rabbi Meir says that this distance is added to the 2,000-cubit Shabbos boundary; the Sages say that this distance was only added between two towns. That is, if adding a karpeif to each city makes their Shabbos boundaries overlap, we allow people to walk between these cities on Shabbos.