As we have seen (section 3), a mekom petur is an area within a reshut ha-rabim like a stone taller than three tefaĥim (about 23 cm) but less than four tefaĥim wide (about 30 cm). Since a mekom petur is not deemed significant, one may transport an object from a reshut ha-rabim or a reshut ha-yaĥid and place it on a mekom petur, and vice versa.
According to Torah law one may move an object from a reshut ha-yaĥid to a reshut ha-rabim by first removing it from the reshut ha-yaĥid and placing it on the mekom petur and then picking it up from the mekom petur and placing it in the reshut ha-rabim. However, the Sages forbade this lest people make light of Hotza’ah that is prohibited by Torah law as well.
Poskim disagree about whether one may use a mekom petur to transfer an item from a reshut ha-yaĥid to a karmelit or vice versa. Some maintain that it is rabbinically prohibited to transfer items by means of a mekom petur even to a karmelit. Even though all the prohibitions of carrying in a karmelit are rabbinic, the Sages did not differentiate here (Ha-Ma’or, Raavad, and Rosh). Others maintain that only when there is a concern that people will end up violating Torah law by carrying from a reshut ha-yaĥid to a reshut ha-rabim or vice versa is it forbidden to transfer items via a mekom petur, but one may transfer them from a reshut ha-yaĥid to a karmelit in this fashion (Rif and Rambam).
In practice, at times of necessity one may rely on those who are lenient and carry from a reshut ha-yaĥid or a karmelit and vice versa via a mekom petur. This is important for soldiers who are stuck in a place with no eruv on Shabbat. If they want to move something from their tent (which is a reshut ha-yaĥid) to the common area of the encampment (which is a karmelit), they should leave their tent and keep moving until they find a mekom petur on which to place the item. Afterward, they can take it from the mekom petur and put it down in the common area, and vice versa. This should also be the procedure for transferring items from one tent to the next via the common area. The item should be removed from the tent, left on a mekom petur in the common area, and then taken wherever it needs to go.
. This is the most practical solution for soldiers in the Israeli army. The best mekom petur is a rock or a pole that is higher than ten tefaĥim (76 cm) and narrower than four tefaĥim (about 30 cm). However, if this pole or rock is lower than ten tefaĥim, as we saw in n. 2, the poskim disagree about whether it is deemed a mekom petur. Some say that there is no such thing as a mekom petur within a karmelit, while others maintain that it can exist. In times of necessity, soldiers in the army may be lenient (Meshiv Milĥama §5 and §60). Sometimes the only way to arrange a mekom petur is by using a bench that is narrower than four tefaĥim . The problem is that another doubt arises in this case, because Me’iri states that a finished product (kli) cannot be considered a mekom petur (SHT 345:15). But in times of necessity when there is no other solution, a kli that is lower than ten tefaĥim in a karmelit can be considered a mekom petur (Sho’el U-meshiv 4:3:2; Hilkhot Eruvin ch. 1 n. 16).A person’s shoulder: According to Rashba and Tosafot, another person’s body can be considered a mekom petur, on condition that it is higher than ten tefaĥim; a person’s shoulder generally fits this condition. Others say that a person’s body cannot be considered a place, and therefore cannot be a mekom petur (see MB 347:10). In times of necessity when there is no other solution, one may rely upon the lenient position.