Abstract: A character signal including sixteen binary coded signal blocks each with an additional bit is stored in a closed loop memory and displayed, as corresponding characters on a display. In response to the display, a student strikes keys on a keyboard to deliver signal blocks, similar in coded form to the stored ones, to a comparator where they are successively compared with corresponding stored signal blocks. Upon the correct striking of the keys, the comparator applies a coincidence signal to the memory to change the value of the additional bit to erase the associated displayed character. Upon incorrect striking of the keys the changed bit is changed back to the original value to again display all the characters on the display.