The person who has physical control of a property has an advantage over anyone claiming rightful ownership.
Other variations (all considered correct): Possession is nine points of the law (nine parts of the law, nine-tenths of the law etc).
Possession is nine-tenths of the law is not a law at all, merely a truism of human nature - that the person who actually holds the item, land or whatever, is in a stronger position than any claimant.
Based on a brief glance at human history, it seems likely that variations on this sentiment would trace back a long, long way, but the earliest written use of the proverb in its modern day form is in Thomas Draxe's Bibliotheca Scholastica (1616): "Possession is nine points of the Law".
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