In the Philippines, the legal status of a child is determined by the marital status of the parents at the time of conception or birth. Children born outside of a valid marriage are classified as illegitimate. Under the Family Code and subsequent remedial laws like Republic Act No. 9255, the process for recognition and the right to use the father's surname has evolved significantly.

I. Understanding Acknowledgment

Acknowledgment is the legal act by which a father recognizes a child born out of wedlock as his own. This act is crucial because, under Article 176 of the Family Code, illegitimate children shall use the surname of their mother unless they are expressly recognized by the father.

Acknowledgment serves two primary purposes:

Filial Rights: It establishes the right of the child to receive support and claim successional rights (inheritance).
Surname Use: It grants the child the legal option to use the father's surname.

II. Modes of Acknowledgment

Acknowledgment can occur through several legal instruments, as provided by the Family Code and the Civil Code:

The Record of Birth: This is the most common method. The father signs the "Affidavit of Admission of Paternity" located on the back of the child’s Birth Certificate (Certificate of Live Birth) at the time of registration.
A Final Judgment: A court order where paternity is proved (e.g., through a compulsory recognition suit or DNA evidence).
Public Document: A separate notarized document, such as an "Affidavit of Admission of Paternity," executed by the father.
Private Handwritten Instrument: A document written and signed entirely by the father in which he expressly acknowledges the child.

III. Changing the Surname (Republic Act No. 9255)

Prior to 2004, illegitimate children were strictly required to use the mother's surname. Republic Act No. 9255, which amended Article 176 of the Family Code, now allows illegitimate children to use the father's surname if the father has recognized the child.

Scenario A: Acknowledgment at Birth

If the father is present at the time of birth and signs the Birth Certificate, the child can immediately be registered under the father's surname. No further court action is required.

Scenario B: Acknowledgment After Birth (Delayed)

If the child was originally registered under the mother’s surname, but the father later wishes to acknowledge the child, the following steps apply:

Execution of Documents: The father must execute an Affidavit of Admission of Paternity (AAP).
Affidavit of Use of Surname of the Father (AUSF): This is a critical document required by the Local Civil Registrar (LCR).

If the child is 0-6 years old, the mother or guardian executes the AUSF.
If the child is 7-17 years old, the child executes the AUSF with the mother's attestation.
If the child is 18 years or older, the child executes the AUSF personally without needing the mother’s attestation.

Registration: These documents are filed with the LCR where the birth was recorded. The LCR will then issue an annotated Birth Certificate.

IV. Rights of an Acknowledged Illegitimate Child

Once acknowledged, an illegitimate child is entitled to the following under Philippine law:

Support: The right to demand financial support from the father.
Legitime: The right to inherit from the father. However, under the Civil Code, the legitime (guaranteed inheritance) of an illegitimate child is generally one-half (1/2) of the legitime of a legitimate child.
Parental Authority: By default, the mother retains sole parental authority (custody) over illegitimate children, even if recognized by the father, as per Article 176. The father usually gains visitation rights unless a court deems him unfit.

V. Key Limitations and Nuances

Irrevocability: Acknowledgment is an admission against interest and is generally irrevocable. It cannot be withdrawn simply because the relationship between the parents soured.
The "Illegitimate" Label: Even if the child uses the father's surname, the child’s status remains "illegitimate" on legal records unless the parents subsequently marry (Legitimation), provided they were not disqualified from marrying each other at the time the child was conceived.
Correction of Entries: If the father's name is already on the birth certificate but the child still uses the mother's surname, a petition for the use of the father's surname can be handled administratively at the LCR via R.A. 9255. However, if the "Father" section is blank, the AAP must be filed first.

VI. Summary Table: Surname Rules

Status of Recognition	Surname of the Child	Required Document
Not Acknowledged	Mother's Surname	None (Default)
Acknowledged at Birth	Father's Surname	Signed Birth Certificate (AAP)
Acknowledged after Registration	Father's Surname (via Annotation)	AAP + AUSF
Parents Marry Later	Father's Surname (Legitimated)	Marriage Contract + Affidavit of Legitimation