Reference for Eruvin 7:1

חַלּוֹן שֶׁבֵּין שְׁתֵּי חֲצֵרוֹת, אַרְבָּעָה עַל אַרְבָּעָה, בְּתוֹךְ עֲשָׂרָה, מְעָרְבִין שְׁנַיִם, וְאִם רָצוּ, מְעָרְבִין אֶחָד. פָּחוֹת מֵאַרְבָּעָה עַל אַרְבָּעָה, אוֹ לְמַעְלָה מֵעֲשָׂרָה, מְעָרְבִין שְׁנַיִם וְאֵין מְעָרְבִין אֶחָד:

An opening (in a wall) between two courtyards, four by four (handbreadths) in the midst of ten (tefachim) from the ground [i.e., if part of it, even a very small part, is ten tefachim from the ground] — two eruvin are made, [each (courtyard) for itself, and each forbids (carrying from one to) the other.] And if they wish, they make one eruv, [these (the men of one courtyard) placing their eruv in the other and being one with them (the men of the other courtyard)]. Less than four by four or higher than ten, two eruvin are made and one is not made. [Less than four by four is not (considered) an opening and is not fit for passing from one to the other. And if it is above ten, too, it does not lend itself to use. Therefore, one eruv is not made. This, only in an unroofed courtyard; but in a roofed house, even higher than ten, one eruv is made, it being customary to place benches and boxes around the house, so that it lends itself to use.]

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