If one wants to walk on Shabbat to a place that lies beyond his teĥum, he can render it permissible by making an eruv teĥumin before Shabbat, that is, by establishing his mekom shevita at the place where he puts the eruv. By placing this eruv, he merges the old teĥum (which would not have allowed him to go where he wants) with the new teĥum (which will allow him to go there) – this is why it is called an eruv (which literally means “merging”) teĥumin. However, the distance that the eruv teĥumin affords him in one direction is lost in the opposite direction. For example, if one places the eruv teĥumin 2,000 amot to the east of his home, he may now walk 4,000 amot eastward (2,000 amot from his home to the eruv and 2,000 amot beyond the eruv), but he may no longer walk westward at all.
There are two ways to shift one’s mekom shevita. The first is by simply walking 2,000 amot in the desired direction before Shabbat begins and staying there for the onset of Shabbat. As long as one is there during the entire period of bein ha-shmashot, that becomes his place, and his teĥum Shabbat is now calculated from that point. He does not need to verbalize anything for this to take effect. It is enough for him to intend to establish his teĥum from that point. In contrast, if one is hiking in a field during bein ha-shmashot but does not intend to establish his mekom shevita there, his mekom shevita remains his home (SA 409:7; MB ad loc. 29).
The second way is to set aside two meals’ worth of food at that place and recite the declaration for making an eruv teĥumin, along with a berakha, as will be explained in the next section. An eruv teĥumin should be made only for the purpose of a mitzva – for example in order to attend a Torah lecture or a mitzva celebration. If one makes an eruv teĥumin for some other purpose, it is still effective be-di’avad (SA 415:1).
When making an eruv teĥumin, one must place it within 2,000 amot of his home. This way his home will be within the teĥum of the eruv, and he may then walk from his home to the eruv. If his home is outside the eruv’s teĥum, the eruv is ineffective, and his teĥum is measured from his home.
One can actually use an eruv teĥumin to travel 5,600 amot, not just 4,000 amot. Since the mekom shevita where he sets aside the eruv is temporary (unlike a city, as above in section 6), he may have in mind for the new teĥum created by the eruv to be oriented so that the square’s diagonal faces his desired direction. He thus gains the additional corners.
. At first glance, it would seem that in most large cities, setting aside an eruv teĥumin is ineffective. After all, we saw in section 4 that when one is outside the city at the start of Shabbat, we do not include the whole city in his four amot. He may travel within the city only as far as his 2,000 amot allow. If so, when one’s home is more than 2,000 amot from the eruv that one makes, the eruv is ineffective, and his status is the same as that of any other resident of the city. Indeed, this is how Beit Me’ir, Maĥatzit Ha-shekel, and Olat Shabbat understand SA 408:1, and so states Eliya Rabba 408:8 as well. According to MA and MB 408:3, 7, 10, SA agrees that one’s mekom shevita in such a case is indeed the location where he set aside the eruv. Since he was in his home when Shabbat began, he may walk within the city in the direction of the eruv, but once he has left the city, he may not return home. According to Rema, since this person’s home is in the city, if he placed an eruv outside the city, he has a connection to both places; therefore, in addition to the 2,000 amot granted him by his eruv, the whole city is considered four amot and he may walk freely within it. Even after he leaves the city, he may return to it and walk within it. BHL’s discussion of this matter concludes with an endorsement of MA’s understanding of SA (408:1, s.v. “raĥok”). However, many rule in accordance with Rema, including Baĥ, Noda Bi-Yehuda (Mahadura Tinyana 49), and AHS. SHT ad loc. 11 states that one should not object to those who are lenient in accordance with Rema. Since the laws of teĥum Shabbat are completely rabbinic, when necessary one may rely on Rema.