A more thorough explanation:

A blank form is a type of document that is used for record-keeping and business purposes. It does not contain any information until it is filled out. Blank forms are not eligible for copyright protection.

For example, a job application form is a blank form. It does not contain any information until the applicant fills it out with their personal details, work experience, and education. Once the form is filled out, it becomes a record of the applicant's job application.

The blank-forms rule is a principle that states that forms are not protectable by copyright if they are designed for recording information but do not convey any information themselves. This rule was established by the U.S. Supreme Court in Baker v. Selden in 1879 and is now a U.S. Copyright Office regulation.