Based on these notions a number of call tutoring systems were developed for the mainframe computers which were used at that time. One of the most sophisticated of these was the Plato system, which ran on its own special PLATO hardware, including central computers and terminals. The plato system included vocabulary drills, brief grammar explanations and drills, and translations tests at various intervals◼ Based on these notions, a number of CALL tutoring systems were developed for the mainframe computers which were used at that time. One of the most sophisticated of these was the PLATO system, which ran on its own special PLATO hardware, including central computers and terminals. The PLATO system included vocabulary drills, brief grammar explanations and drills, and translations tests at various intervals