The concept of constitutional construction is of central importance to originalist theory but is both underdeveloped and controversial among originalists. Some object that its apparent open-endedness undermines the constraining virtues of originalism and exposes citizens to arbitrary judicial power. In this Article, we respond to this challenge by presenting an originalist theory of constitutional construction that can guide and constrain judicial activity within the “construction zone.” When combined with an originalist theory of constitutional interpretation, our approach yields a unified theory of originalism. Our theory of constitutional construction draws upon a familiar common-law concept long used in contract and fiduciary law to han...