The Weberian concept of closure, the concerted collective action aimed at excluding rival groups from competition for economic opportunities and resources, has captured the attention of sociologists studying stratification and social inequality for decades. Closure has been suggested as a cause of intergroup inequality across professional, ethnic, religious, and national boundaries, among others. However, most studies applying the concept have ignored a basic distinction drawn by Weber between closing the market to competitors and closing the group to outsiders. This inattention has not only been responsible for conceptual confusion but also threatens to undermine the usefulness of the concept in understanding intergroup inequality. The aim of this paper is twofold. First, it re-examines Weber’s definition of closure and shows that in its original formulation, market closure is different from group closure. Second, it argues that making this conceptual distinction is essential for disentangling two phenomena that are equally capable of producing inequality among groups. Apart from a brief exegesis of Weber's account of closure, a simple computer-simulated agent-based model (ABM) is offered to illustrate how market closure and group closure, combined with individual competition, are independently sufficient to bring about an unequal distribution of resources among groups. The message for empirical researchers using closure as an explanation of inequality is clear: failing to draw the distinction between closing the market and closing the group will, at best, lead to causal indeterminacy or, at worst, to false causal conjectures.