The spatial prepositions in and on apply to a wide range of containment and support relations, making exhaustive definitions difficult. Theories differ in whether they endorse geometric or functional properties and how these properties are related to meaning and use. We directly examine the roles of geometric and functional information in adults’ and children’s use of in and on by developing a large sample of relations situated within a small gradable geometric and functional feature space. We propose that variation in features across items is systematically related to the use of in and on and demonstrate that feature-language relationships change across development: adults’ expression use is sensitive to both geometric and functional features, while children’s use varies only according to geometric features.