Short answer. Under Civil Code Article 468, the thing of greater value is treated as the principal. If the two things are equal in value, the one with the greater volume becomes the principal. In painting, sculpture, and printed works, the material support is always the accessory.
What the law says
the thing of the greater value shall be so considered, and as between two things of equal value, that of the greater volume
Civil Code, Article 468 — Principal by Value or Volume. Read the full provision →
The two-step rule for joined things
When two things are joined together and neither qualifies as a mere ornament or accessory of the other, the Civil Code sets a clear order for deciding which one controls. First, look at value: the thing worth more money is the principal. If both things happen to be worth the same amount, the law moves to a second test — volume. The bulkier or larger of the two becomes the principal, and the other follows it as the accessory.
Why the distinction matters
The label 'principal' is not merely descriptive. Whoever owns the principal thing is entitled to the combined result under the rules on adjunction. The owner of the accessory loses the separate ownership of that item and instead acquires a right to be compensated for its value. Getting the classification wrong — or failing to document the relative values at the time of union — can mean losing a claim to the merged object entirely, or forfeiting a valid compensation claim against the other party.
The special rule for creative works
Article 468 carves out an important exception for artistic and written works. In painting and sculpture, writings, printed matter, engraving and lithographs, the law reverses the usual logic: the canvas, the paper, the board, the metal, or the parchment is always treated as the accessory — no matter how much it costs or how large it is. The creative work itself is the principal. This means an artist who paints on an expensive canvas does not lose the artwork simply because the material support is worth more; the painting dominates.
Practical steps when two things are joined
If you are dealing with a situation where two objects have been physically united — construction materials embedded in a structure, a gemstone set in metal, a part fused to a machine — the first practical step is establishing each item's fair market value at the time of incorporation. Documentary evidence of purchase price, appraisal, or replacement cost strengthens your position. Where values are genuinely equal, shift the analysis to volume or physical extent. The outcome of this classification determines who takes the whole and who is owed compensation.