Why do we need ro'in by square posts. A mechitza does nit have a shiur in thickness.
Why not just say that the square post itself is one ama worth of wall in one direction-
(And the same in the other)
Is there some thing abiut it being a "double L shaped board" that matters? Is that doraissa or drabonon
Best Regards,
Michael Jacob Rubin
1) The Ramban writes, in Milchamos Hash-m on page 6a in the Rif pages DH Kosuv, that with Diyumdin (Pasei Biroyos) we need to say Ro'in; we look at the Pasei Biroyos as if they are longer than they really are and they reach other and close up. This is also written by Rashi Sukah top 7b DH Pasei; that we look at it as if the mechitzah goes from one end to the other on the side of the well.
2) Pasei Biroyos themselves only need to be 1 amah long, but the length around the well could be several amos long. For instance the square area around the well might be 10 amos by 10 amos, whilst the Pasei are only 1 amah each. Therefore, this would seem to be considered as Porutz Merubah Al HaAmud; the open space is greater than the closed wall; so the area around the well would be considered as open. What ro'in does is to say that the closed mechitzah is greater than the open space between them, so the area around the well is closed.
3) The Ritva, at the very beginning of the second chapter of Eruvin, cites an opinion that Parutz Merubah is only a derabanan problem, whilst Mideoraisa even Parutz Merubah is Reshus Hayachid if there is a proper Mechitzah. According to this it might be possible to say that according to the above Milchamos and the Rashi in Sukah, ro'in is only required miderabanan to remove a derabanan problem of Porutz Merubah, but mideoraisa the Pasey Biroyos are valid without ro'in. This requires further study.
KOL TUV
Dovid Bloom
Follow-up reply:
1) The opinion that I cited above in 3) that Parutz Merubah is derabanan is in fact the opinion of the Raavad, cited by Chidushei Rashba 15b DH Hachi Agmerei. So we can argue that the need to say that we lengthen the mechitzos of Pasei Biroyos is only required miderabanan and ro'in is sufficient to achieve this.
2) Tosfos 17b DH Osin does in fact write that since there is an amah worth of wall in each direction this is considered as a Petach. Reb Chaim Brisker, on the Rambam Hilchos Shabbos 16:16 DH veHineh Kasheh, explains that since the amah is considered as a Petach it follows that it is no longer a Pirtzah. Parutz Merubah is only a problem if it is a Pirtzah not if it is a Petach.
Behatzlachah Rabah
Dovid Bloom