FamilySearch Indexing: Philippines, Metropolitan Manila, Manila—Civil Registration of Birth, 1901–1979, FAQ
Question: It has come to my attention many are asking about the "middle name" of the children. We can tell by the mother if the middle name is the same as her surname; the question is whether we index the child's two surnames or put the mother's portion of the surname in the Given Names field?
Answer: The child does not have two surnames. The name that matches the mother's last name is the child's middle name and should be indexed as part of the Given Name in the Given Names field. The name that matches the father's last name is the Child's surname and is indexed in the Surname field. If the child is illegitimate then the name that matches the mother's middle name is also the child's middle name and indexed with the Given Name. The child's surname is the same as the mother's surname and indexed accordingly.
Philippines Rule for Names:
The mother's maiden name is the child's middle name. The child's father's surname is the child's surname.
If the child is illegitimate the mother's middle name and surname are the child's middle name and surname.
(This information should be under the Project Updates tab instead of hidden under the FAQ tab. Who would think to look under FAQ to see a project update?)