Well, what about the side post? TB Eiruvin 8

Today’s daf TB Eiruvin 8 discusses a disagreement whether one may carry up to the cross beam (koreh- קורה) or may one carry underneath the cross beam. A lot depends on what you consider the purpose of the cross beam is. Two suggestions are provided describing the purpose of the cross beam. One possible explanation is that the cross beam serves as a conspicuous marker (משום היכר). Another possible explanation is that the cross beam serves as a wall or a partition (משום מחיצה). Within each of the explanations the disagreement can be explained. If the purpose is a conspicuous marker, is this marker for the people inside the alleyway as they exit it or is it for the people in the public domain entering the alleyway? If this koreh serves as a partition, which edge of the beam does one use, the interior edge or the exterior edge?

 “A dilemma was raised before the Sages: What is the halakha with regard to utilizing and carrying in the area beneath the cross beam spanning the opening of an alleyway, which the beam permits carrying? Opinions differ on the matter. Rav, Rabbi Ḥiyya, and Rabbi Yoḥanan said: It is permitted to utilize the area beneath the cross beam. Shmuel, Rabbi Shimon bar Rabbi, and Rabbi Shimon ben Lakish said: It is prohibited to utilize the area beneath the cross beam.

 “The Gemara suggests a way to understand this dispute: Shall we say that these amora’im argue over the following issue, that Master, representing those who permit it, holds: A cross beam serves in an alleyway as a conspicuous marker that separates it from the public domain, and Master, representing those who prohibit it, holds: A cross beam serves as a partition.

 “The Gemara rejects this argument: No, everyone might agree that a cross beam serves as a conspicuous marker, but here they argue over the following: Master, representing those who forbid it, holds that the conspicuous marker is intended for those situated inside the alleyway, and hence the area outside the inner edge of the cross beam may not be used; and Master, representing those who permit it, holds that the conspicuous marker is intended for those outside in the public domain, and it is therefore permitted to carry up to the outer edge of the cross beam.

 “The Gemara proposes an alternative explanation: And if you wish, you can say that everyone agrees that a cross beam permits carrying as a partition, and here they argue over the following issue: As one Sage holds that the inner edge of the cross beam descends to the ground and seals off the alleyway, and therefore under the cross beam is not within the closed-off area; and the other Sage holds that the cross beam’s outer edge descends to the ground and seals off the alleyway, and therefore it is permitted to carry even in the area beneath the cross beam. Consequently, there is no need to connect the dispute with regard to utilizing the area beneath the cross beam to the dispute with regard to the nature of the cross beam.” (Sefaria.org translation)

 All this should sound familiar. The halakha follows Rabbi Yoḥanan; consequently, one may carry underneath the koreh. (Shulkan Arukh, Orekh Hayim, 364:4) At the end of this sugiya, Rav Ḥisda teaches us something brand-new. If the alleyway is adjusted by a side post (lekhi- לחי), up to which edge may one carry?

 “Rav Ḥisda said: All concede that utilizing the area between the side posts placed at the entrance to an alleyway to permit carrying is prohibited, for a side post functions as a partition, and therefore one may only use the space up to its inner edge, but no further.” (Sefaria.org translation)