Tally Definition Language is the application development language of Tally. TDL is developed to provide the user with flexibility and power to extend the default capabilities of Tally and integrate them with the external applications. TDL provides a development platform for the user. The entire User Interface of Tally.ERP 9 is built using TDL. TDL as a language, provides capabilities for Rapid Development, Rendering, Data Management and Integration. TDL is an Action driven language based on definitions. It emphasizes strongly on the concept of reusability. It comprises of Interface and Data objects. Interface Objects mainly determines the behavior of the product in terms of user experience. Data objects are mainly used for data persistence in the Tally Database. Any user of Tally.ERP 9 can learn TDL and develop extensions for the product. The entire source code of the product is available as part of the Tally Development  environment i.e. with our product Tally Developer.