Eruvin, Chapter 5, Mishnah 1. This mishnah presents two concepts relating to measuring the Shabbos boundary outside a city: ibbur ha'ir (the extension of the city) and ribua ha'ir (squaring the city).
Ibbur Ha'ir (Extension of the City):
The term 'ibbur ha'ir' means the pregnancy of the city - an addition that protrudes beyond the rest of the city. Any house located within seventy amos and two thirds of the city's edge is included in the city and is considered an addition (ibbur) to it. And if within that same distance of seventy amos and two thirds there is another house, the extension continues further out, and the city's boundary is measured only from outside its extension. As long as there are houses linked to one another by that same interval, they are considered a continuation of the city. (In the mishnah we will see that besides a house there are other things that count for this purpose.)
The mishnah opens with a question: "Keitzad me'abrin es he'arim?" - how does one create the extension, and how is the addition to the city determined?
And it answers: "Bayis nichnas bayis yotzei" - the boundary line at which the city's houses end is not even; rather, one house is set back inward and another juts outward. And even where there is no actual house, the following are also treated as houses:
"Gedudiyos gevohos asarah tefachim" - walls that remain from a house that collapsed, where the walls are still ten tefachim high or more.
"Ugesharim unefashos she'yeish bahen beis dirah" - bridges, and also the monuments and markers erected over a grave, that have a dwelling place in them - a spot for a watchman to sit. Since there is living space connected to these places, they are all considered houses.
"Motzi'in es hamidah kenegdan" - the measurement is extended out to correspond to the place where they stand. If one of them protrudes, for example at the northeastern corner, the city's measurement will begin from that point.
Ribua Ha'ir (Squaring the City):
"Ve'osin oso kemin tavla meruba'as, kedei sheyihei nischar es hazaviyos" - the city is squared off like a square board, in order to gain the additional corners.
In other words: if at the northeastern corner of the city an expansion was created by one of these things - a house, or one of those items that contains a dwelling place - then that corner of the city has been extended outward. And even though on the southeastern side there is nothing of the sort, we square the city and draw an imaginary line down from that northeastern corner, as though there were a house at the southeastern corner opposite it as well, and in this way the city is squared. From that point outward we measure the two thousand amos of the city's boundary.
In summary: in this mishnah we learned two concepts in measuring the city's boundary. Ibbur ha'ir - a house (and likewise remnants of walls ten tefachim high, bridges, and grave monuments that contain a dwelling place) located within seventy amos and two thirds of the city's edge joins to the city, and so on from house to house, and the measurement is extended out to correspond to it. Ribua ha'ir - the city is made into a square board, and the corners are filled in even where there is no house there, and from the line of the square we measure the two thousand amos.