Short answer. When two requisites of Article 1016 of the Civil Code are met: two or more persons were called to the same inheritance, or the same portion of it, without division into specific shares, and one of them died before the testator, renounced the inheritance, or was incapacitated to receive it.
What the law says
In order that the right of accretion may take place in a testamentary succession, it shall be necessary: (1) That two or more persons be called to the same inheritance, or to the same portion thereof, pro indiviso; and (2) That one of the persons thus called die before the testator, or renounce the inheritance, or be incapacitated to receive it.
Civil Code, Article 1016 — Requisites in Testamentary Succession. Read the full provision →
The first requisite: being called together, undivided
Article 1016 requires, first, that two or more persons be called to the same inheritance, or to the same portion thereof, pro indiviso. This means the will must have named more than one person to share the same inheritance or the same specific portion of it, without the testator dividing that portion into separate, individual shares for each of them. If the testator instead assigned distinct, separate portions to each heir individually, this first requisite for accretion is not satisfied at all.
The second requisite: one of them fails to inherit
The second requisite is that one of the persons thus called die before the testator, or renounce the inheritance, or be incapacitated to receive it. Any one of these three events, predecease, renunciation, or incapacity, creates the vacancy that accretion is meant to address. Without one of these specific triggers, there is no lapsed share for the surviving co-heirs to absorb in the first place, regardless of how the inheritance was originally structured.
Why both requisites have to be present together
Article 1016 requires that it shall be necessary that both conditions exist, meaning accretion in testamentary succession does not happen from either requisite alone. Two or more people being called together to an undivided portion, without any of them later failing to inherit, gives no vacant share to accrete. And a lapsed share among heirs who were never called to the same undivided portion in the first place does not trigger accretion either, since there is no group of co-heirs positioned to absorb it.
What this means for where a lapsed share ends up
When both requisites are satisfied, the vacant share generally passes to the co-heirs who were called with the person who failed to inherit, rather than falling back into the general estate to be distributed under the ordinary rules of succession. Confirming both elements, that the heirs were called pro indiviso to the same inheritance or portion, and that one of them actually died, renounced, or was incapacitated, is what determines whether this accretion rule under Article 1016 applies to a particular lapsed share.