QUESTION: The Beraisa describes the manner in which one may stand in a garden (which is a Karmelis) and use a key to open the lock to the garden when the lock itself is a Reshus ha'Yachid. According to Rebbi Meir, one must be able to stand in a Reshus ha'Yachid next to the lock while he uses the key. He may not stand in the garden (a Karmelis) and place the key in the lock (a Reshus ha'Yachid), lest he return the key to the Reshus in which he stands (a Karmelis), which is prohibited.

RASHI (DH Sha'arei) explains that the lock can have the status of a Reshus ha'Yachid if it is four by four Tefachim wide, and it is at least ten Tefachim above the ground.

The law is that the Mechitzah of a Reshus ha'Yachid must reach within three Tefachim of the ground. If it does not, it is a "Mechitzah sheha'Gediyim Bok'in Bah" (14a, 16b) and it is disqualified as a Mechitzah (according to everyone except Rebbi Yosi b'Rebbi Yehudah (Shabbos 5a, Eruvin 33b)). A lock that is ten Tefachim high does not have Mechitzos that reach within three Tefachim of the ground. Why, then, is it considered a Reshus ha'Yachid?

(a) The TOSFOS HA'ROSH suggests that perhaps the Gemara refers to a lock that stands on a pedestal which reaches the ground and is four by four Tefachim wide (or even three by three Tefachim wide, according to TOSFOS to 33a, DH v'Chalkalah).

(b) Alternatively, the Tosfos ha'Rosh suggests that there is a mound under the lock that is at least three Tefachim high. This mound prevents goats from passing easily underneath the lock, and thus the issue of "Mechitzah sheha'Gediyim Bok'in Bah" does not apply. (In such a case, even the Rabanan who argue with Rebbi Yosi b'Rebbi Yehudah agree that the lock is a Reshus ha'Yachid; see Tosfos to 33a, DH d'Hadar).

(c) The RITVA answers that the garden's lock is underneath an awning that extends into the garden from the wall of the garden. This awning is four Tefachim (or more) wide and extends four Tefachim (or more) from the wall. Since it is considered a "ceiling," the principle of "Pi Tikrah Yored v'Sosem" applies. Each edge of the ceiling is viewed as if it extends to the ground and forms a proper Mechitzah. The problem of "Gediyim Bok'in Bah" does not affect a Mechitzah created through the principle of "Pi Tikrah Yored v'Sosem" (see Insights to Eruvin 25:2).