PL/CT is a special version of PL/C designed to permit programs to be run interactively from a typewriter terminal. It is completely compatible with normal PL/C - that is, the source languages accepted by PL/C and PL/CT are identical and the results of execution are exactly the same. Hence, a program can be developed and tested under PL/CT and subsequently run under normal PL/C (or vice versa). PL/CT permits the user to interact with the program during its execution. Output will be printed on the terminal and input data may be requested from the terminal. The course and rate of execution can be controlled from the terminal. It is also possible to interrupt execution and display and alter the values of variables. However, the source program itself cannot be changed under PL/CT. PL/CT receives a complete program, compiles it, and then executes it in interactive mode. But to make any change in the program it is necessary to leave PL/CT, make the change under the CMS editor, and then present the modified program to PL/CT for complete recompilation. This Guide provides only minimal information about CMS, perhaps sufficient for very straightforward programming tasks.