The need to travel from place to place stems from man’s deficiency: he cannot find his livelihood and meet his needs by remaining stationary. So he must roam and leave his place. But the idea of Shabbat is for every Jew to rest from his travails and worries, contemplate the inner perfection of creation, thank God for choosing Israel from all peoples and giving us His Torah, and delight in God and His goodness.
For this reason, the Sages ordained boundaries (teĥumin) within which one may walk on Shabbat, and beyond which one may not walk. One’s teĥum Shabbat (Shabbat boundary) is determined by his mekom shevita – the place where he is spending Shabbat (literally “stopping place”) – plus 2,000 amot in each direction (2,000 amot corresponds to 2,000 steps of an average person, approximately 912 m).
If one is spending Shabbat in a field (i.e., not in a city or town), his mekom shevita is defined as four amot squared. This is the amount of space one occupies when lying on the ground with his arms and legs outstretched. He may walk 2,000 amot beyond that in each direction. If he is spending Shabbat in a city or town, the entire settled area is considered one place, and he may travel 2,000 amot beyond the city limits (see section 8 below).
According to Rambam and Smag, teĥum Shabbat restrictions are based on Torah law, though the Torah prohibition forbids one to travel more than twelve mil (24,000 amot, which is 10944 m, almost eleven km) from his mekom shevita. This is based on the size of the Israelite camp in the wilderness, as the Torah states: “Let everyone remain where he is; let no man leave his place on the seventh day” (Shemot 16:29). However, according to Ramban, Rosh, Rashba, and most Rishonim, this verse does not refer to the laws of teĥum but rather to those of carrying in a reshut ha-rabim; all boundary restrictions (even beyond twelve mil) are rabbinic law.
The Sages ordained that if one must travel beyond the teĥum on Shabbat – in order to attend a wedding celebration or a Torah lecture – he may make an eruv teĥumin, which extends his teĥum in the necessary direction (as explained below in sections 12-14). One who leaves his teĥum forfeits it and must remain within his four amot (as explained below, section 11).
. According to SA 396:1, one’s “place” is four amot by four amot, as it is with regard to carrying in a reshut ha-rabim or a karmelit. Rema writes that some maintain for the purposes of teĥumin, one’s place extends four amot from the center in each direction, for a total of eight amot by eight amot.
. The Sages were uncertain whether boundary restrictions apply above ten tefaĥim; perhaps, since people do not walk there, the prohibition does not apply there. In practice, when there is uncertainty pertaining to a rabbinic prohibition, the law follows the more lenient position. Therefore, regarding boat travel in oceans and rivers, where Rambam agrees that the prohibition is not a Torah prohibition (as such travel is not comparable to travel in the wilderness), as long as one is ten tefaĥim above the seabed, the teĥum prohibitions do not apply (Eruvin 43a; SA 404:1). According to SA 248:2, the ten tefaĥim above the seabed are measured from the bottom of the boat, while according to R. Eliezer of Metz they are measured from one’s feet. SAH and MB 248:14 state that in times of need one may be lenient. However, if one is traveling at a height of more than ten tefaĥim above land, several poskim maintain that one should be stringent in accordance with Rambam and not travel more than twelve mil (Rema 404:1).