Inosc is a habit tracking application for iPhone designed for use by two people. Unlike conventional habit apps that focus on personal streaks, Inosc pairs two individuals who each maintain their own set of habits while working toward a shared goal. Each person selects five habits and designates one as a priority. A day is counted when habits are completed, with the priority habit carrying double weight, for a total of four possible points per day. Users pair by exchanging a code, and each person's check-ins and photos appear on the other's device. If one person misses a day, the other can send a nudge. The app uses a visual metaphor of two trees with a shared root system. Each person's habits keep their tree in leaf, and days when both people complete their habits cause the roots to grow toward each other. After 66 days of mutual participation, the roots meet. The 66-day period is based on research by Lally et al. (2010) at University College London, published in the European Journal of Social Psychology, which found that a new habit typically takes about 66 days to feel automatic and that missing a single day did not undo progress. If a user misses several days, their tree turns amber and rests, but it does not die, and the app does not issue negative messages. Users may also start the app solo and invite a partner later. The app's design is informed by research on habit formation and the influence of a partner's behavior on health behavior change.