Short answer. By scaling everyone down. Article 853 says that where the instituted heirs' aliquot parts together exceed the whole estate, each part is reduced proportionally. Nobody is dropped and no share is singled out; every heir's fraction shrinks by the same ratio until the shares fit the estate the testator actually had.
What the law says
If each of the instituted heirs has been given an aliquot part of the inheritance, and the parts together exceed the whole inheritance, or the whole free portion, as the case may be, each part shall be reduced proportionally.
Civil Code, Article 853 — Shares Reduced Proportionally. Read the full provision →
What Article 853 provides
Article 853 provides that if each of the instituted heirs has been given an aliquot part of the inheritance, and the parts together exceed the whole inheritance, or the whole free portion, as the case may be, each part shall be reduced proportionally. Testators over-dispose more often than you would think — a half here, a half there, a third to someone else, and the fractions add up to more than one whole. The article does not treat that as a contradiction that voids the will. It rescales the shares so they fit, keeping every instituted heir in the picture.
Proportional reduction, not a race to claim
The reduction is proportional, so the ratios the testator set between his heirs are preserved even as the absolute shares fall. Suppose he gives one heir a half and another three-quarters — five-quarters in all, a quarter more than exists. Each share is scaled down by the same factor, so the half becomes two-fifths and the three-quarters becomes three-fifths. The heirs still stand in the original two-to-three relationship, and together they now take exactly one whole. No heir is preferred and none is dropped; the excess is absorbed evenly across all of them.
Why the law rescales instead of failing the will
The alternative approaches would all defeat the testator's plan. Voiding the over-generous will would send the estate to intestacy the testator plainly did not want. Paying the shares in the order written until the estate ran out would reward whoever was named first and cheat the rest. Cutting one arbitrary share would pick a loser for no reason found in the will. Proportional reduction is the only method that treats every instituted heir alike and keeps faith with the balance the testator struck. It reads the excessive fractions as evidence of how he wanted the estate shared, not as a fatal defect.
Where the legitime comes in
Reduction among instituted heirs is one thing; protecting compulsory heirs is another, and the two must be kept straight. The free portion is what the testator may distribute as he likes, and Article 853 scales the instituted shares to fit whatever pool they were drawn from — the whole estate, or the free portion, as the case may be. Shares reserved by law for forced heirs are not part of what gets reduced to accommodate over-generous gifts. So identify the free portion first, apply the proportional reduction inside it, and leave the legitime of the compulsory heirs untouched by the arithmetic.