Recent questions led me to discussions of theoretical keys, which are defined by Wikipedia as keys with at least one double accidental in the key signature. (Unfortunately, the source of that definition is not given.)
The key signatures on Wikipedia (and most other sources I've seen) put the double accidental in the spot where the single accidental would go. In other words, for G♯ major, the F𝄪 is in the spot where F♯ would normally go (i.e., at the beginning). This, I admit, seems intuitive to me:
Although the majority of online sources notate it that way, at least one other source puts the double sharp at the end. But this is hardly the greatest source, so I was skeptical that this notation would be correct. Imagine my surprise when I saw that LilyPond's default behavior (version
2.19.80) seems to notate it the same way:
While I can understand the logic of this latter key signature—the last added accidental appears last in the key signature—it's odd to see a key signature that begins with a C♯. (With that said, I guess it's no more odd than seeing a key signature with an F𝄪!)
Is there an official rule for notating key signatures with double accidentals? Or is there no rule since these are just "theoretical" entities?
Note that, according to this discussion among the MuseScore community, the top key signature (with the F𝄪 first) was custom made. The LilyPond key signature, however, exists by default.