Reference for Eruvin 5:2
נוֹתְנִין קַרְפֵּף לָעִיר, דִּבְרֵי רַבִּי מֵאִיר. וַחֲכָמִים אוֹמְרִים, לֹא אָמְרוּ קַרְפֵּף אֶלָּא בֵין שְׁתֵּי עֲיָרוֹת, אִם יֵשׁ לָזוֹ שִׁבְעִים אַמָּה וְשִׁירַיִם, וְלָזוֹ שִׁבְעִים אַמָּה וְשִׁירַיִם, עוֹשֶׂה קַרְפֵּף לִשְׁתֵּיהֶן לִהְיוֹתָן כְּאֶחָת:
A karpef (an enclosed space) is provided for a city. [Whoever comes to measure t'chumin for a city leaves a space of seventy cubits and a fraction, seventy and two-thirds cubits; and from there he begins to measure the two thousand cubits, viz. (Numbers 35:4): "From the wall of the city and outwards, one thousand cubits roundabout," Scripture hereby intimating: Provide an "outwards," and then measure, i.e., provide a karpef of seventy cubits and a fraction and then measure from there.] These are the words of R. Meir. The sages say: A karpef was instituted only between two cities. If each had (an outer edge of) seventy cubits and a fraction, a karpef is made for both, to be as one. [If two cities were close to each other, each is accorded a karpef of seventy cubits and a fraction in order to connect them thereby, to be regarded as one city; so that if one wished to go from one of them through the other, two thousand cubits are measured for him outside of the other, the two of them being regarded as one city through these karpefoth which join them. The halachah is in accordance with the sages, that a karpef is not provided for one city but only for two.]