Short answer. When two movable things from different owners are united in good faith into one object, the owner of the principal thing becomes the owner of both. However, the law requires that owner to pay the former owner of the accessory the full value of what was absorbed.
What the law says
the owner of the principal thing acquires the accessory, indemnifying the former owner thereof for its value
Civil Code, Article 466 — Adjunction (Conjunction). Read the full provision →
What adjunction means and when it applies
Adjunction — sometimes called conjunction — is the civil law term for what happens when two distinct movable things owned by different people become permanently united into a single object. Article 466 of the Civil Code sets the rule: the owner of the principal thing acquires the accessory but must indemnify the former owner for its value. The provision applies only where neither party acted in bad faith. If bad faith is involved, the rules shift significantly and the innocent party has additional remedies.
How to tell which thing is the principal
The Civil Code provides criteria in subsequent articles to identify which item is the principal and which is the accessory. Generally, the thing that determines the purpose, character, or use of the combined object is the principal. Where that test is inconclusive, the one of greater value controls; if values are equal, the one of greater volume prevails. Getting this right matters because it determines who ends up owning the entire combined object and who is entitled only to monetary compensation.
The indemnity obligation is mandatory
Acquiring the accessory is not a windfall — it comes with a legal obligation to pay. The former owner of the accessory does not walk away empty-handed; the law entitles that person to the value of the absorbed item. Value is measured at the time of union. If the parties cannot agree on the amount, the question becomes one of fact to be resolved with evidence of what the thing was actually worth. This indemnity obligation exists regardless of which party initiated the joining.
Good faith is the threshold condition
Article 466 begins with the phrase without bad faith. The good-faith rule protects an owner who did not know or could not have known that the thing being united belonged to someone else. If bad faith enters the picture — for instance, if one owner deliberately incorporated another person's property knowing it was not theirs — the innocent owner has the option to demand separation if it is still possible, or to claim the value of the property taken, along with damages. The good-faith framework of Article 466 is therefore only the starting point of a larger set of rules on adjunction.