a teaching aid for piano and other keyed musical instruments . the device has switches electrically connected to a matrix array and mechanically linked to each key for detecting each key depression , an led associated with each key for signalling the keys which should be played and an alphanumeric display for showing notes in literal and octave notation and for showing the student &# 39 ; s position in a composition or exercise . the displays are connected in a matrix array and along with the key sensing switches are connected through interface circuitry to a microcomputer . the microcomputer stores a sequence of musical steps forming a composition and compares played notes to the stored sequence . when the compared notes and the depressed keys are identical , the student is signalled and the microcomputer advances to the next musical step . when there is an error , the microcomputer recycles the same musical step . an external , auxiliary memory , such as a cassette tape , may be used for storage of data representing a variety of musical compositions . suitable programming permits the apparatus to be used for preparing and storing lessons , such as by the manual input at the instruments keyboard of additional musical compositions . unique interface circuitry provides improved led display , minimizes circuit components and permits the key sensing switches and the displays for an 88 - key instrument to be linked to a modern microprocessor through only 13 input / output terminals , including only 8 terminals for data transfer and 5 for addressing and control .