As we have seen, by squaring the city, we add space in the corners to the teĥum. However, the Sages pointed out that sometimes we cannot draw straight lines to square the whole city, because the resulting square would include uninhabited areas that are too large to be considered subsumed by the city. Examples of this would be cities that are shaped like a bow or like an L. The operative principle is that if there are 4,000 amot between the two ends of the shape, that part of the city cannot be squared.
An L-shaped city:
A bow-shaped city: