RobinX: a three-field classification and unified data format for round-robin sports timetabling. Sports timetabling problems are combinatorial optimization problems which consist of creating a timetable that defines against whom, when, and where teams play games. In the literature, sports timetabling problems have been reported featuring a wide variety of constraints and objectives. This variety makes it challenging to identify the relevant set of papers for a given sports timetabling problem. Moreover, the lack of a generally accepted data format makes that problem instances and their solutions are rarely shared. Consequently, it is hard to assess algorithmic performance since solution methods are often tested on just one or two specific instances. To mitigate these issues, this paper presents RobinX, a three-field notation to describe a sports timetabling problem by means of the tournament format, the constraints in use, and the objective. We use this notation to classify sports timetabling problems presented in the operations research literature during the last five decades. Moreover, RobinX contains xml-based file templates to store problem instances and their solutions and presents an online platform that offers three useful tools. First, a query tool assists users to select the relevant set of papers for a given timetabling problem. Second, the online platform provides access to an xml data repository that contains real-life problem instances from different countries and sports. Finally, the website enables users to interact with a free and open-source C++-library to read and write xml files and to validate and evaluate encoded instances and solutions.