If no parents survive but grandparents on both the father's and mother's side do, how is the reserved half divided between them?

Division by line, not by head

The rule is that if the testator leaves neither father nor mother, but is survived by ascendants of equal degree of the paternal and maternal lines, the legitime shall be divided equally between both lines. That is a division by side of the family, not among the individuals. Two surviving grandparents on the father's side and one on the mother's side do not take a third each. The paternal pair share one half of the reserved portion between them, taking a quarter apiece, while the lone maternal grandmother takes the other half entire.

Equal degree is the condition, and it is checked first

Everything above assumes the survivors stand at the same remove from the deceased. Grandparents are all second degree in the ascending line, so a set drawn from both sides usually qualifies. Where they do not, the article is blunt: if the ascendants should be of different degrees, it shall pertain entirely to the ones nearest in degree of either line. A surviving grandmother on one side therefore excludes great-grandparents on the other altogether. The nearer relative does not merely take more — the more distant one takes nothing, and the equal-division rule never comes into play.

There is no representation upwards

The most common misconception here is that a grandparent inherits through the deceased parent, taking whatever that parent would have taken. Succession in the ascending line does not work that way. Grandparents inherit in their own right, as the nearest surviving ascendants, which is why the shares are worked out line by line rather than by tracing the dead parent's place. It also means a deceased parent's own siblings, your aunts and uncles, take nothing on that theory.

What the reserved half leaves over

The portion being divided is one-half of the estate, the legitime of the ascending line where no children or descendants survive. The other half is the free portion, and a surviving spouse's fourth and any illegitimate child's legitime are taken from it, not from the grandparents' share. In practice the first step in any of these estates is to fix the family tree as of the date of death — who was alive, on which side, at what degree — because the entire computation turns on that and on nothing else.

Related provisions

• Civil Code, Article 890 — Legitime Of Legitimate Parents
• Civil Code, Article 889 — Legitime of Legitimate Parents