Key Number (also called Note Number) refers to the piano-key numbering used in the Musical Instrument Digital Interface (MIDI) standard. Key Numbers range from 0 to 127. A standard 88-key piano uses Key Numbers from 21 (A0) through 108 (C8).
In GeoShred, a Key means any playable rectangle in the Performance area where touching it plays a note. A particular Key on the screen can correspond to a variety of Key Numbers, depending on the Octave setting and the Keyboard Layout.