(Mishnah): If one was traveling and feared lest it get dark [on Erev Shabbos], and he saw a fence or tree and said 'my Shevisah is under it', his words have no effect [since he did not specify a particular place];

If he said 'my Shevisah is at the trunk', he may walk 2000 Amos to the tree, and another 2000 Amos from the trunk to his house, 4000 Amos in all after dark.

If he did not see a fence or tree, or did not know the Halachah, and said 'my Shevisah is where I am', he gets 2000 Amos in every direction, i.e. a circle [of radius 2000]. R. Chanina ben Antiganas says so;

Chachamim say, he gets a square [of side 4000], he gains the corners. (He may walk 2000 Amos north [or south], and additionally 2000 east [or west].)

(Gemara): If [two are together, and] one of them knows [a fence or tree], and one does not know, the one who knows goes with the one who does not know, and the one who does not know goes with the one who knows (he intends to be Shoves in the place that his friend knows and declares to be his Shevisah).