An English maxim has it that fences make good neighbors. In biblical times, territorial borders were marked off with boundary stones. Typically, a boundary-stone landmark might be one stone set up on end, indicating the border between a man's field and his neighbor's.
You shall not move your neighbor's boundary mark, which the ancestors have set, in your inheritance which you will inherit in the land that the LORD your God gives you to possess. (Deuteronomy 19:14)
During the settlement of American territories, a similar method was used. Settlers would set up rocks or drive in stakes to indicate parcels of land that they were claiming. Hence the idiom "staking a claim." Often it happened that in their absence unscrupulous neighbors or other settlers would remove these landmarks to their own advantage.
Boundary markers worked the same way in the biblical era. An unscrupulous neighbor might move a boundary stone and steal a hundred feet of your field. According to the prophet Hosea, God pours out His wrath like water on those who move boundary stones (Hosea 5:10).
The Torah places a special curse on someone who moved a boundary stone, saying, "Cursed is he who moves his neighbor's boundary mark" (Deuteronomy 27:17). The Proverbs warn against moving boundaries and especially against encroaching on the boundaries of widows and orphans:
Do not move the ancient boundary which your fathers have set. Do not move the ancient boundary or go into the fields of the fatherless, for their Redeemer is strong; He will plead their case against you. But He will establish the boundary of the widow. (Proverbs 22:28, 23:10-11, 15:25)
The prohibition against moving a boundary stone can be applied to many situations in life. It reminds us that God deems it healthy and appropriate to maintain proper boundaries and distinctions. For example, boundaries that maintain a distinction between genders are important.
A person should always be careful to protect the boundaries between private life and public life, between family and friends, between parent and child, between husband and wife, and so on. When boundary lines become fuzzy, confusion and conflict ensue. The godly person is careful to maintain a sense of where another person's property and privacy ends and begins.
There are four types of people. The one who says, "What's mine is mine and what's yours is yours." This is the normal type of person, but some say this is the type of person who lived in Sodom. The one who says, "What's mine is yours and what's yours is mine." This type of person is an ignoramus. The one who says, "What's mine is yours and what's yours is yours." This is a righteous person. The one who says, "What's mine is mine and what's yours is mine." This is a wicked person. (m.Avot 5:10)