Reference for Eruvin 5:1
כֵּיצַד מְעַבְּרִין אֶת הֶעָרִים, בַּיִת נִכְנָס בַּיִת יוֹצֵא, פָּגוּם נִכְנָס פָּגוּם יוֹצֵא. הָיוּ שָׁם גְּדוּדִיּוֹת גְּבוֹהוֹת עֲשָׂרָה טְפָחִים, וּגְשָׁרִים וּנְפָשׁוֹת, שֶׁיֵּשׁ בָּהֶן בֵּית דִּירָה, מוֹצִיאִין אֶת הַמִּדָּה כְנֶגְדָּן, וְעוֹשִׂין אוֹתָהּ כְּמִין טַבְלָא מְרֻבַּעַת, כְּדֵי שֶׁיְּהֵא נִשְׂכָּר אֶת הַזָּוִיּוֹת:
How me'abrin cities? ["me'abrin," as in "ishah me'ubereth" (a pregnant woman). This is the intent: How are cities extended? "If one house were recessed and another projected etc." When he came to designate the city tchum and to measure two thousand cubits outside it, if the wall were not straight, but the houses were close together…] If one house were recessed [within the city more than its neighbor, so as to give a "defective" appearance] and another projected [more than its neighbor], or if one turret were recessed and another projected, or if there were tall ruins [sections of the wall of ruined houses within seventy cubits and a fraction of the city], or structures over tombs (provided that they contain living quarters) — the measure is taken opposite them [i.e., If the projections were in the northeast corner, it is perceived as if there were other projections opposite it in the southeast corner, and a line stretched from one to the other; and the measurement is taken from the line outwards so that the tchum be equal in the two corners and not longer in one and shorter in the other.] And it [the tchum] is made like a square tablet, [two thousand cubits on the sides as in the middle] so that the corners be gained, [and (it is not made) round, two thousand cubits in the middle and losing at the sides, as a circle does.]