The Historic Charter of Barcelona is a research project published by means of an online tool, which integrates detailed knowledge about the city history and makes it available to citizens and professionals. The Historic Charter of Barcelona is a tool to explain the history of the city through the cartographies that describe it. It contains 26 maps that have been drawn from texts, historical cartographies, archaeological maps, modern cartography and contemporary digital maps. A committee of historians specialising in the various historical eras have cross-checked and corrected the documents under the direction of the Barcelona City History Museum (MUHBA). The project also features new contents and geolocates unpublished working documents as the topographical plan that Ildefons Cerdà used to design the Eixample. The documentary sources include maps and aerial photographs from the Cartographic Institute of Catalonia, the Barcelona City History Museum, the Carta Arqueològica de Barcelona, Barcelona Historical Archive, the Cadastre and the Municipal Institute of Informatics (IMI). The redrawing not only facilitates the review and correction of errors in the old cartographies, less accurate, but also provides a graphic representation that is consistent throughout all the stages. This coherence allows the cartographies to be real in parallel and compare the transformations experienced by the city. The tool is public and is online. It allows superimposing maps of various ages, incorporating different information -original working documents and other non-morphological data- and drawing on top of the plans. Thanks to these interaction capabilities, the Historic Charter is a great instrument for communication, pedagogical support and research purposes. The project has a dual dimension: both the content and the cartographic processing/publishing tool, made-to-measure using open source software, which implements a new methodology to streamline and update the information fluently (bug fix, incorporation of new discoveries and historical periods). The Historic Charter of Barcelona is a pioneering tool in the research and dissemination of urban history, making Barcelona the first city in the world to have a tool of this complexity. The Historic Charter is a document aiming to study the history but also to explain it: a tool to think the future by understanding the past.