Eruvin1: 5
הָיְתָה שֶׁל קַשׁ וְשֶׁל קָנִים, רוֹאִין אוֹתָהּ כְּאִלּוּ הִיא שֶׁל מַתֶּכֶת; עֲקֻמָּה, רוֹאִין אוֹתָהּ כְּאִלּוּ הִיא פְשׁוּטָה; עֲגֻלָּה, רוֹאִין אוֹתָהּ כְּאִלּוּ הִיא מְרֻבַּעַת. כֹּל שֶׁיֵּשׁ בְּהֶקֵּפוֹ שְׁלֹשָׁה טְפָחִים, יֵשׁ בּוֹ רֹחַב טֶפַח.
If it was [made] of straw or of reeds, we view it as if it were [made] of metal; bent, we view it as if it were straight; rounded, we view it as if it were squared. Anything which has a circumference of three fists has a diameter of one fist.