Eruvin 10a and Eruvin 10b, finishing up the issue of nir’eh mi-bahutz ve-shaveh mi-bifnim, whether walls visible from outside an area but not inside can count as walls, thence to how wide R. Yehudah would have allowed an opening of a mavoi to go, with the contrast case of pasei bira’ot, a four cornered area Hazal allowed for drawing water from a well, and then to how we close off extra space of a mavoi entrance, depending on atei avira de-hai gisa u-de-hai gisa, air on both sides nullifying a post, and omed merubbah al ha-parutz, whether solid material has to be more than empty space to nullify it, or just equal to it.
