Short answer. Article 788 of the Civil Code favors the reading that keeps the disposition operative. When a testamentary provision admits of different interpretations, the interpretation under which the disposition actually takes effect is preferred over one that would leave it void or without effect, whenever there is genuine doubt about which reading is correct.
What the law says
If a testamentary disposition admits of different interpretations, in case of doubt, that interpretation by which the disposition is to be operative shall be preferred.
Civil Code, Article 788 — Interpret So the Will Operates. Read the full provision →
The tie-breaker favors giving effect to the will
Article 788 supplies a specific rule for genuinely ambiguous clauses: if a testamentary disposition admits of different interpretations, in case of doubt, that interpretation by which the disposition is to be operative shall be preferred. Between a reading that would make the clause work and a reading that would leave it void or ineffective, this article directs the court toward the reading that keeps the testator's disposition alive.
The rule only kicks in where there is real doubt
Article 788 is triggered by the clause admitting different interpretations and there being genuine doubt about which is correct — it is not a general license to reinterpret a will however a party finds convenient. Where the language is actually clear, there is nothing for this preference to operate on; the rule exists specifically for the situation where a provision can honestly be read more than one way, and the court needs a principle for choosing between them. Article 789 sets a related limit on how far a court may go to cure an ambiguity: extrinsic evidence may be considered when uncertainty appears on the face of the will, but the testator's own oral declarations of intent are expressly excluded — the court works from the document and its context, not from what someone recalls the testator saying they meant.
Why the law leans toward validity rather than nullity
The underlying logic of this article is that a will represents a testator's considered wishes, and a testator who wrote a clause almost certainly intended it to have some effect rather than none. Reading an ambiguous clause in the way that voids it would, in most cases, defeat that underlying intent more than a reading that lets it operate. Article 788 turns that general instinct into an actual rule of construction rather than leaving it to guesswork.
What matters when you are arguing over a clause like this
Lay out both possible readings of the disputed clause clearly, and be explicit about which one would make the disposition operative and which would not — that contrast is exactly what Article 788 asks a court to weigh. Keep the full will and any related documents on hand as well, since the ambiguous clause is read in the context of the whole document, not in isolation from the rest of what the testator wrote.