Common ownership is the talk of the town in antitrust land. The competitive implications of rival firms being partially owned and controlled by a small set of overlapping owners are both fascinating and hotly contested. Could the source of potential harm be minority shareholder control in a setting of widely held companies? Critics question the extent and mechanisms of common owners’ influence driving any pro- or anticompetitive effects. This chapter aims to present a comprehensive account of partial ownership, capturing the incentives and effects of both individual and institutional investors and also cross- and common shareholding. It illustrates the early historical unity between corporate and competition laws in regulating shareholding acquisitions but also their progressive quiet disconnect. Triggered by the contemporary common ownership (hypo)thesis, it puts forward a taxonomy of shareholding types and their control characteristics from a competition law perspective, with emphasis on commonly thought passive and diversified investment holdings. The chapter concludes by urging competition and corporate governance and finance policymakers towards harmonic regulatory solutions to address common ownership. It also offers a quantum theory of the corporate property “atom”, drawing cautionary tales about the dynamic and ambiguous qualities of minority common shareholding for antitrust enforcers.