95:5 There are various leniencies utilized in measuring the Shabbos boundary. These should only be employed by an expert.
95:6 If one needs to travel beyond the Shabbos boundary on Shabbos or yom tov, he must make "eiruvei techumin" before Shabbos or yom tov begins. One does this by placing his eiruv within the city limits in a place where he is permitted to travel. The place where he deposits the eiruv is now considered his "home base," from which he has 2,000 cubits in every direction. This does not extend his Shabbos boundary; it shifts it so that what he gains on the side where he put his eiruv, he loses on the other side. For example, if he put the eiruv 2,000 cubits to the east, he may not travel at all to the west because he is already distant from his home base by the length of the Shabbos boundary.