Short answer. The painting is the principal thing; the canvas is the accessory. Article 468 of the Civil Code specifically provides that in painting, the board, metal, stone, canvas, paper, or parchment is deemed the accessory. Ownership consequences flow from this classification.
What the law says
In painting and sculpture, writings, printed matter, engraving and lithographs, the board, metal, stone, canvas, paper or parchment shall be deemed the accessory thing.
Civil Code, Article 468 — Principal by Value or Volume. Read the full provision →
The general rule for determining which thing is principal
When two things are incorporated and it is not immediately clear which is the principal and which is the accessory, Article 468 provides a hierarchy of criteria. First, compare value: the thing of greater value is the principal. If values are equal, look to volume: the thing of greater volume is principal. These criteria apply to most cases of adjunction or combination. However, Article 468 then carves out a specific exception for certain creative works that overrides this general analysis.
The creative-work exception: the medium is always accessory
For paintings, sculptures, writings, printed matter, engravings, and lithographs, Article 468 imposes a categorical rule: the physical medium — canvas, board, metal, stone, paper, or parchment — is always deemed the accessory thing. It does not matter how valuable the canvas is, or how much larger it is than the artwork applied to it. The law treats the intellectual and artistic work as the principal and the material on which it is executed as the accessory. This reflects the priority the law gives to creative expression over the mere material support.
Why this classification matters
Classifying something as principal or accessory determines ownership when two things belonging to different people are combined. The accessory follows the principal — meaning the owner of the principal thing generally becomes the owner of the combined object. In the painting scenario, the artist who created the work and whose painting is the principal thing has a strong claim to ownership of the combined canvas-and-painting, subject to whatever obligations arise toward the canvas owner. Practically, it means the canvas owner cannot simply claim the painting as their own just because they own the material it was painted on.
Rights of the canvas owner
Even when the canvas is classified as accessory, the canvas owner does not simply lose their property without any remedy. Civil Code rules on adjunction generally provide for compensation to the owner of the accessory thing, particularly when the union was made in bad faith by the artist who used someone else's materials without permission. The canvas owner may be entitled to the value of the canvas, plus damages if the artist acted in bad faith. Good faith or bad faith on the part of the painter matters significantly in determining the extent of compensation the canvas owner may claim.