RELAX: A software platform for PDE interface relaxation methods. RELAX is a system designed for solving partial differential equations (PDEs) on composite domains, especially, for modeling mechanical objects which are formed from a heterogeneous set of parts where, unlikely, any one modeling scheme would be sufficient. The RELAX system for programming interface relaxation techniques for PDEs which uses multiple geometric models is not a geometric modeling system per se. It is oriented towards domain-independence, that is, the system tries to make as few assumptions about the PDEs domains as possible. RELAX seems to be a system most suitable as a rapid prototyping tool allowing the user to sketch the composite domains on the screen, rather than a production solution.