Bava Batra 28 - Presumptive Ownership of Real Estate - Chazakah
When a person seeks to exact property from another one, the burden of proof is on the claimant. In other words, it is logical to presume that the one who currently holds the object in his possession is the rightful owner.
In the case of real property, physical "possession" of it is impossible. Current occupancy is also inconclusive, since it it impossible to prevent people from ever entering one's property. Rather, presumptive ownership is assigned to the last person known to have held the property. Thus, when that person claims that the current occupant is an illegal squatter, the burden of proof is on the occupant. If, however, the current resident established a "chazakah," usually by continuous uncontested occupancy for three years, the burden of proof is shifted to the previous owner.