In English, some words sound the same but are distinguished by silent written markers of their morphological status, as in plurals (e.g., "cities"), singular possessives (e.g., "city's") and plural possessives (e.g., "cities'"). We examined undergraduates' ability to correctly omit or position apostrophes in these three word types before and after a 3-week intervention that focused on the conceptual bases for plurality and possession, as well as their associated spelling rules. Pre-test spelling was imperfect for plurals and poor for possessives, especially plural possessives. Abstract understanding of plurality and possession, measured in a morphological awareness task, was also relatively poor. The intervention resulted in significant but imperfect improvement on all tasks, with post-test morphological awareness predicting post-test spelling of plurals and possessives, even after controls for general spelling ability. Control participants showed no change. We suggest that adults' difficulty in spelling possessives stems from a shaky grasp of the underlying morphological concepts, and we discuss possibilities for future intervention.