(Mishnah - R. Meir): One may not stand in Reshus ha'Yachid and unlock [using a key] in Reshus ha'Rabim, or stand in Reshus ha'Rabim and unlock in Reshus ha'Yachid, unless [in the latter case] he made a Mechitzah 10 Tefachim tall [around the door and stands inside the wall].

Chachamim: A case occurred in a butcher's market in Yerushalayim, in which people would lock and leave the key in the window over the door!

R. Yosi: It was a market of wool merchants.

(Gemara - R. Acha citing R. Chinena citing Kahana): The Halachah does not follow R. Yehudah (i.e. Chachamim).

Question (R. Aba bar Papi): How does R. Meir [apply Chokekim Lehashlim] - is it k'Gimum (as if above that place was cut off), or k'Nakuv (as if a hole was carved out)?

If you will say k'Gimum, it is only above 10 (if it is below 10 and one cut off everything above, no Reshus ha'Yachid remains). If you will say k'Nakuv, even below 10 (it is like a hole of Reshus ha'Yachid. We explained this based on SEFER NIR.)

(Beraisa): Openings to gardens that have a Beis Sha'ar (a small house to which the opening opens) inside, one opens and locks from the inside. [If the Beis Sha'ar is] outside, one opens and locks from the outside;

[If it has a Beis Sha'ar] here and here (inside and outside), one may open and lock here and here. [If it does not have] here or here, he takes the key and opens and locks and leaves it in its place.

What is the case? The lock is above 10 Tefachim, but if the lock is below 10, he takes the key from the porch, opens or locks and leaves it in its place. R. Meir says so;

Chachamim say, even if the lock is above 10 Tefachim, he brings the key from Erev Shabbos, puts it on the porch, [and on Shabbos he] opens, enters and moves it in the house, locks and leaves it in the hole above the lintel (HA'GAON RAV C. KANIEVSKY, SHLITA).

If the hole was four Tefachim it is forbidden, for we may not transfer from Reshus to Reshus through a Reshus (Mekom Petur).

Inference: R. Meir makes it k'Gimum. (When it is above 10, he forbids taking from the porch to the lock, for he views the lock like a hole of Reshus ha'Yachid. This is based on SEFER NIR above.)

Question: Granted, [if he wants to open or lock] from the outside, a Beis Sha'ar is needed [lest he transfer from Reshus ha'Rabim to Reshus ha'Yachid]. From the inside, is a Beis Sha'ar needed?! (Also the garden is a Reshus ha'Yachid mid'Oraisa, just mid'Rabanan we consider it Karmelis because it is not Hukaf l'Dirah!)

Answer: We do not find two Reshuyos using one Reshus [without an Eruv, even if both are Reshus ha'Yachid, lest people say think that likewise, one may transfer between Reshus ha'Rabim and Reshus ha'Yachid. The same concern applies here - PNEI MEIR.]