Short answer. Article 315 describes it: taking undue advantage of the signature of the offended party in blank, and writing a document above that signature, to the prejudice of the signer or of a third person. It is a form of estafa, and the prejudice is part of what has to be shown.
What the law says
By taking undue advantage of the signature of the offended party in blank, and by writing any document above such signature in blank, to the prejudice of the offended party or any third person.
Revised Penal Code, Article 315 — Swindling (Estafa). Read the full provision →
The provision in full
Article 315 lists, among the frauds committed with unfaithfulness or abuse of confidence, the act of taking undue advantage of the signature of the offended party in blank, and by writing any document above such signature in blank, to the prejudice of the offended party or any third person. Three things are needed. The signature must have been given in blank. A document must have been written above it. And prejudice must have followed, either to the signer or to somebody else. The provision sits under abuse of confidence because it presupposes the sheet was entrusted.
"Undue advantage" is the element in dispute
Signing a blank sheet is not by itself the offence, and people do it for ordinary reasons — leaving a form to be completed with agreed details, signing ahead of a transaction that a colleague will finish. What the article punishes is exceeding what was entrusted. Where the signer authorised the other person to write particular content and that is what was written, the case is very different from one where a wholly different instrument appeared above the signature. So what was agreed at the moment of signing is the fact everything else turns on.
How the document itself gives evidence
A document written above an existing signature often carries physical traces: text crowded or unusually spaced to fit the available room, alignment that does not match the signature, differences in ink or in the printing, a body and signature made at different times. These are matters for examination of the original, which is why obtaining the original rather than a photocopy matters so much. Alongside that sits the surrounding record — why the sheet was signed, who kept it, and what the parties said to each other before and after.
What to do, and what not to do again
If a document like this has surfaced, preserve everything as it stands and do not annotate or correct it. Secure the original if you can, note who has had custody of it, and gather the messages showing what the blank sheet was for. Then take advice quickly, because a document in circulation can be acted on by third parties while the dispute is unresolved. The general lesson is plainer than the law: a signature given in blank is a signature given to whatever is written above it later.