An extension that protrudes under a window is referred to by the Mishna on our daf as a ziz. (A ziz and a gezuztra are similar projections from a window. According to the Geonim, a ziz has supports beneath it, while a gezuztra is, in effect, a balcony, with walls that extend from the building above it). The Mishna teaches that a person can place objects on this ziz, or take things from the ziz into the house on Shabbat. The Gemara limits this rule in a number of ways. For example, according to the Gemara, the protrusion must be above ten tefahim from the ground. As we have learned before, the rules of reshut ha-rabim – of the public domain – only extend up until ten tefahim. Above that is a makom petur – a “free space” that is neither public nor private. If it is larger than four tefahim square, then it becomes a reshut ha-yahid (private domain) that extends from the house.
If you say that the ledge protrudes into a public domain, one should be prohibited to place an object on it, as we should be concerned lest the object fall and he will forget and come to bring it in from the public domain to a private domain. Rather, it must be that the ledge protrudes into a private domain; but if so, it is obvious that it is permitted to place objects on it and to remove them.
Abaye said: Actually, the mishna is dealing with a case where it protrudes into a public domain, and what is the meaning of that which it teaches: One may place objects upon it? This refers to fragile utensils, which would break instantly if they fell. Consequently, there is no concern that one might then bring them in from the public domain to the private domain.
Rashi explains that if such items as plates or cups made of glass or earthenware fall down, they will break, so there is no concern that they will be carried back into the house.
Rabbenu Yehonatan gives another explanation for permitting breakable utensils to be placed on the ziz. He argues that people will be particularly careful with their breakable items and will make sure that they do not fall to the ground. Since they are so careful, the likelihood that someone will come to carry them becomes less of a concern.