APL2/X is an interface between APL2 and the X Window System, built at the IBM Cambridge Scientific Center. This interface enables the full set of the X Window System Xlib calls and the related data structures to be used directly from programs written in APL2, thereby providing APL2 with a true, full-function windowing environment. The interface also deals with the broader and more general issue of how to call C programs from APL2. Some differences exist due to the very different nature of C and APL2. Important differences include: 1. Function arguments are always specified by a value in the same way that they would be for regular APL2 functions. 2. Only input parameters may be specified on the calls to the X Window System. 3. All results from calling a function are returned as explicit results, including results returned in C via changes to the output arguments.