sentences:  In experimental high energy physics, several levels of computing are used to simulate data runs, including programs that generate matrix elements and ones that generate events.However, there are several programs for each of these tasks, such as CompHEP and MadGraph to generate matrix elements, and PYTHIA and HERWIG for event generation.Depending on specific properties of the particle decay that physicists are interested in, they may desire to use a certain program for these tasks, but before the Les Houches Accords, there was no general interface for communicating between the programs.