For 220 years, the Patent Act has required patentable inventions to be “new and useful.” For almost as long, courts have struggled with difficult questions about what to protect by patent and what to leave in the public domain. Patent laws provide a variety of rules to help answer these questions, but the very nature of translating an inventive principle into written form can make application of these rules elusive. As a result, consistent application of patent law can often involve guesswork and hand-waving at the margins of difficult cases. There are many principled ways to make headway through such doctrinal thickets. Some look to economic analysis, others to natural rights, while still others focus primarily on patentability criteria such as newness and disclosure. In the broad scheme of things, however, the requirement that an invention be useful has been nearly non-existent – essentially ignored. The level of “utility” currently required to obtain a patent is extremely low: the invention need only operate as described and potentially provide some de minimis public benefit. Patents that fail to meet this standard are rare, and usually limited to perpetual motion machines and chemicals with unknown effects. As a result, the invention’s usefulness has never been considered an over-arching principle that can help decide difficult questions about what should be patented. Ignoring usefulness is a mistake. Usefulness can be – and in some ways already is – surprisingly helpful in patent law and policy. This article’s goal is to show how usefulness is not only relevant, but also how it is important. Usefulness can underlie a principled approach to resolve longstanding doctrinal questions, resolve close factual questions of patentability, explain existing rules, or provide meaning to vague requirements. The doctrine is especially helpful at the margins, where some policy is needed to decide close cases; usefulness can often put a thumb on the scale. To be sure, usefulness does not answer every question, and in some cases can only provide guidance rather than resolution. Even so, this article follows on its predecessor, Reinventing Usefulness, by presenting the first comprehensive illustration of how some difficult, fundamental questions become answerable when viewed through the lens of usefulness. The analysis is descriptive in part and normative in part. Sometimes rules implicitly or explicitly consider usefulness, and some rules should start doing so.