Residential alleys are ubiquitous urban features in many cities, commonly defined as narrow streets used for service access. Intuitively, alleys are not recognized as having clear purpose as public open spacesOpen space because we move on sidewalks, recreate in parks, unwind in backyards, cultivate in community gardens, and play on playgrounds. However, by taking a close look at residential alleys in multiple cities, this paper posits that alleys are potentially the most functionally diverse and inclusive public open spacesOpen space. Despite their utilitarian designation, alleys are used for commuting, recreation, gardening, tinkering, socialising, artistic and political expressions, leisure, play, and much more. The wide range of activities is enabled by the unique morphological properties of alleys. They are simultaneously small fragments of large networksNetworks and self-contained spaces of distinctive morphological assemblages. Residential alleys unsettle conventional assumptions of how public open spaceOpen space is produced. Social and morphological configurations of alleys are not created through professional plans and design. Instead, they emerge through tension between formal and informal spatial practices, benign neglect by local governments and purposeful reappropriations by citizens, and the privacy of residential homes and disorder of urban life.