rə is an open source and free-to-use suite of tools that lets users create a historical map of the world. It is based on open source software from OpenStreetMap and other projects, and is similar to OpenStreetMap in that it relies on users to contribute and maintain geographic data. rə is specifically designed to include a time component with all data, so the resulting database represents the state of the world throughout history, rather than at just one particular time such as the present. Read more about system architecture and design in our HICSS-54 paper "Kartta Labs: Collaborative Time Travel". rə is a community-driven effort. As users contribute more data, rə grows to represent more areas of the world and more times throughout history. The initial release of rə includes extensive data for New York City and Washington, DC throughout 19th and 20th centuries. rə is an open source project by Google.