The first perek often uses g'di'in bokiin as a limitation of l'vud to 3 tefachim. While I see how this limitation is relevant to RaShBaG's view thta l'vud is 4 tefachim, I am not sure I understand how this applies to Chachamim.
Since Chachamim hold L'vud is only up to 3 tefachim anyway, why do I need g'di'in bokin?
Many thanks, in anticipation
Shabbat shalom
Marc Abrahams, Ramat Beit Shemesh, Israel
Without Lavud, there would still be the possibility to say another concept which we find by Mechitzos (and which is in fact used in certain Rabbinic cases where the Chachamim saw a reason to be lenient), namely Gud Achis (that a wall is considered as if it extends downwards).
When there is a ten Tefach wall which is more than three Tefachim above the ground, the Mechitzah would be ruled to continue to the ground if it were not for Gedi'im Bokim Bo. (Of course, the concept of Lavud is a concept which applies in almost all situations, whether or not there are Mechitzos.)
Kol Tuv,
Yaakov Montrose