37 CFR 1.34: Acting in a representative capacity.

When a patent practitioner acting in a representative capacity appears in person or signs a paper in practice before the United States Patent and Trademark Office in a patent case, his or her personal appearance or signature shall constitute a representation to the United States Patent and Trademark Office that under the provisions of this subchapter and the law, he or she is authorized to represent the particular party on whose behalf he or she acts. In filing such a paper, the patent practitioner must set forth his or her registration number, his or her name and signature. Further proof of authority to act in a representative capacity may be required.