The classic Just-In-Time (JIT) manufacturing system is known for its need for a frozen schedule, balanced work load and inability to handle fluctuations in demand. But today research and development are addressing ways for a JIT system to accommodate greater fluctuations. One approach is to design the JIT system with machines that have a greater flexibility in what work they can perform. An initial step is the development of flexibility measures so that different system configurations and control mechanisms can be easily compared. This paper presents a background to, and a rational for, a flexibility measure for a JIT system.