Theoretical key

In music theory, a theoretical key is a key whose key signature would have at least one double-flat () or double-sharp ().

Some musical keys are not normally used because they would require a double sharp or double flat in the key signature. For example, G♯ major requires eight sharps, and, since there are only seven scale tones, one tone requires a double sharp. The enharmonically equivalent key of A♭ only requires four flats, making it clearer to read.