Laurence Tratt, the designer of the Converge language, has written a detailed introduction to RPython, the language used as the base of the PyPy project. "However, in addition to outputting optimised C code, RPython automatically creates a second representation of the user's program. Assuming RPython has been used to write a VM for language L, one gets not only a traditional interpreter, but also an optimising Just-In-Time (JIT) compiler for free. In other words, when a program written in L executes on an appropriately written RPython VM, hot loops (i.e. those which are executed frequently) are automatically turned into machine code and executed directly. This is RPython's unique selling point, as I'll now explain."