Who are forced heirs under the law?

Article 807 of the Civil Code of 1889 provides:
Article 807. The following are forced heirs:

1. Legitimate children and descendants, with respect to their legitimate parents and ascendants;

2. In default of the foregoing, legitimate parents and ascendants, with respect to their legitimate children and descendants.

3. The widower or widow, natural children legally acknowledged, and the father or the mother of the latter, in the manner and to the extent established by Articles 834, 835, 836, 837, 840, 841, 842, and 846.