sci The XPA messaging system provides seamless communication between many kinds of Unix programs, including X programs and Tcl/Tk programs. It also provides an easy way for users to communicate with these XPA-enabled programs by executing XPA client commands in the shell or by utilizing such commands in scripts. Because XPA works both at the programming level and the shell level, it is a powerful tool for unifying any analysis environment: users and programmers have great flexibility in choosing the best level or levels at which to access XPA services, and client access can be extended or modified easily at any time.