11 pagesDespite its usefulness for overcoming floating-point arithmetic defects, manipulating imprecise data, and performing non-linear global optimization, interval arithmetic is not yet a first-class type in any computer language. Some attempts are underway for Fortran and Java, not for C++. However, some add-on C++ interval libraries already exist though they do not respect the C++ standard library spirit since they are not generic (one cannot choose the type for the interval bounds). The design and implementation of JAIL, a C++ templated library based on the ``traits'' C++ programming technique, are presented in this paper; parameterization is shown to enhance the reliability, the versatily, and the portability of the library across mul...