Abstract: This essay reviews The Origins of the Necessary and Proper Clause — a book by Gary Lawson, Geoffrey P. Miller, Robert G. Natelson, and Guy I. Seidman. In the book the authors contest the conventional wisdom that there is no firm historical basis for understanding the Necessary and Proper Clause. Professor Natelson contends that the clause picked up incidental powers principles of agency law, including a general requirement of reasonableness and more detailed fiduciary duties of impartiality, good faith, and due care. Professors Lawson and Seidman trace the Necessary and Proper Clause to traditions of English administrative law that imposed “reasonableness” requirements on official decision-makers. Finally, Professor Miller reads the clause in light of analogous language in eighteenth-century corporate charters that reflected a reasonably close means-ends requirement and an anti-discrimination principle. Rather than try to draw any definite conclusions about the meaning of the Necessary and Proper Clause, the essay examines, more generally, how to make sense of the nitty-gritty details of the private or public law backgrounds of an important constitutional clause. First, starting from the premise that interpreters should ask what legal conventions a “reasonable” lawmaker might be expected to know, the essay explains why we might care about the private or public law backdrops to the clause even if we have no proof that any of those frameworks subjectively influenced a constitutionally sufficient majority of ratifiers. Second, the essay differentiates, for interpretative purposes, between adopting a term of art and merely borrowing an off-the-rack legal construct from a particular legal context. When drafters borrow a widely used legal construct (such as an incidental powers clause), it may not make sense to import all of its associated obligations into a new and different area of law, especially when the same type of construct is common to many diverse legal frameworks. Third, the essay suggests a way for thinking about the burden of persuasion when legal scholars uncover lost constitutional meanings, as the book does.