35 U.S.C. § 121 — Divisional applications

If a patent application claims two or more inventions that are independent and distinct from each other, the Director of the Patent and Trademark Office may require the applicant to narrow ("restrict") the application to just one of those inventions. If the applicant then files the other invention as a separate "divisional application," and that application meets the requirements of section 120, it gets to keep the benefit of the original application's filing date. Once the Director has required this kind of restriction — or once an application has been filed as a result of that requirement — a patent that issues from one of these applications cannot later be used as a reference against the other one, either in the Patent and Trademark Office or in court. This protection applies to both the divisional application and the original application (and any patent issued on either), as long as the divisional application was filed before the other application's patent issued. Finally, a patent's validity can't be challenged just because the Director failed to require this kind of restriction.