Noun
theoretical key (plural theoretical keys)
(music theory) a key with key signature which exceeds 7 sharps or 7 flats. In music using equal temperament its pitch coincides with its enharmonic
G-sharp major is a theoretical key with 8 sharps; it is enharmonic to A-flat major.
The scale F-flat major is a theoretical key with 8 flats; it is enharmonic to E major.