Electronic musical instrument generating supplementary notes automatically established from played notes

A three-keyboard electronic musical instrument of the type generating binary-coded primary key data representative of notes played on the keyboards, and secondary key data representative of unplayed notes that are correlated with the played notes in some musically favorable way. Prior to introduction into a multi-channel tone generator circuit, part of the primary key data and all of the secondary key data are directed into a key data memory having storage channels corresponding in number to the sounding channels in the tone generator circuit available for such data. A discrimination data memory has a plurality of storage locations, corresponding to the storage channels of the key data memory, for storing binary-coded data representing the discrimination between the primary and the secondary key data stored on the corresponding storage channels of the key data memory. In response in part to the output from the discrimination data memory a memory control circuit causes the key data memory to store the primary key data in preference to the secondary key data when the key data memory has no empty channel for each incoming data, by invalidating a secondary key data already in storage.