Slide rule, 'Musigraph 100', for music theory, plastic, in vinyl sheath with printed instructions (see part/stroke records), by Musigraph Company, Teaneck, New Jersey, United States of America, 1985

Invented in 1971, the Musigraph was marketed to universities and colleges, symphony orchestras, rock groups and chamber ensembles to assist with theory and harmony. This side enables all the notes in the scale to be found, given the tonic or keynote, for each of nine ‘modes’. The Ionian is our normal major scale, the Aeolian the ‘natural’ minor. The Lydian has a raised fourth note; the Mixolydian is major with a flattened seventh note, and so on. Other scales on the rule give the position of the keynote should you wish to modulate to a new key, and all chord possibilities for any particular key.