Early (French?) baroque composers sometimes used a non standard form of key signature:
This is taken from Marin Marais (Pièces De Viole-Livre-II). Marais used it almost systematically in this book, for 1, 2, or 3 sharps. He never used it for flats, but he had very few keys with flats anyway, and the French composers of the time had the habit of removing one flat from the keys for minor pieces, which seems to be a misguided inheritance of Guido D’Arezzo’s Gamut. Michel de la Barre also did this in his suites, but not so systematically, though he did not balk at doing it even for 4 flats! I have not examined all of such occurrences, I think the idea is to display the accidental for every note which is either adjacent to one of the staff lines, or sits on one.
Is there a possibility of doing this with Dorico? On one hand, when transcribing baroque music, you might feel motivated to reproduce exactly the composer original score. In a beautiful ScoringNotes tutorial, @claude_g_lapalme showed us it was possible to do a lot even as early as Dorico 2. But I did not see an example of this there.
Though I am generally all for notation fidelity to baroque original, I can live without this one for “normal” s ores. But I have also be experimenting with @FredGunn’s Monster staff. For this one, you feel a little lost if you have only the accidentals one in the middle. Of course, having all of their possible occurrences might be overwhelming in F minor! Currently we get:
I am only experimenting on this, so it is more a whim that a need! But the faithful reproduction of originals remains a motivation. Anyway, if this is currently possible, great, else I guess this is just a “nice to have”.
Is this what you’re looking for?
The sharps are half of their normal size to keep them from touching when they are an interval of a fourth apart.
Here is the project:
Legacy baroque key signatures.dorico (527.2 KB)
John, could you briefly summarise the procedure? Do you use a personalised key signature or symbols ?
I would also be interested to know how to obtain an “intavulatura” (i.e. a staff with 6 and 8 lines, as in certain first Frescobaldi editions : see below).
Wow 
!
Yes, this is exactly what I was looking for.
And yes, I can see you added CustomKeySignatures to the Equal temperament (12-EDO) TonalitySystem. Well knowing, first that it can be done, and second where you should go to do it, I will give a shot at it!
Thank you so much.
I edited the 12-EDO tonality system and created three new accidentals with pitch delta one:
- An accidental with two small sharps in one column which provides the F# in G major and the G# in A major.
- An accidental with three small sharps in one column which provides the F# in D and A major.
- An accidental with no glyph which provides the C# in D and A major.
I then created three custom key signatures for G, D and A major using these accidentals.
It is easy to create staves containing six and eight lines by using Library > Instruments. However, getting the clefs and key signatures where you want them on these staves is not currently supported by Dorico’s user interface.
I used @FredGUnn’s Monster Staff as an experiment, and it looks ok:
I have not yet tried the customized key signatures on it, but I shall !
Thanks, John, for the explanations about French baroque time signatures.
And, alas, I note your observation for the second point. 
Just to add: this habit of writing key signatures is not an especially french style, here is some writing from the Bach family:
Yes, but this case, Bach imitates the French style, inevitably.
What else ? 
Ok:
JSBach, Art of Fugue - personal manuscript, beginning:
(Yes, there is one of the many fugues in there called „In stylo francese“ (sic), but not this one)
JSBach, Orgelbüchlein - Vom Himmel hoch
Not a french style piece
It‘s just a very common way to notate in this time
It took the emerging tonal minor mode a bit of time to “settle in” in musical practice.
I don‘t think, that this is a question of mode … the Art of Fugue example is in d minor, the Orgelbüchlein example in D major.
The whole thing started centuries earlier with fa-signs at the beginning of pieces, to remind the singer to use the proper hexachord combinations. I guess our modern habit to notate key signatures came about sometime in the middle of the 18th century, and most of our notation practices are 19th century and later.
See also, how Bach cancels the c sharp in the Orgelbüchlein example with a flat sign (the old fa hinter)
I’m using the word in the sense of the (“modern”; i.e. mid/late–17th century on) tonal “major mode” and “minor mode.” Obviously Bach’s practice is tonal, and indeed eventually helped us later write “the book” on tonality. But it is true that it took some time for the notational practice of minor key signatures to be codified.
BTW, in the baroque era there were two ways to relate the minor to the major note: the older one starting on the second note (re) of the major mode, and the other one (which won this race) starting an the 6th degree of the major scale. (The first one was often called „dorian“ later on). There were some discussions between maestri still in the first half of the 18th century
(Nicholas Baragwanath, The Soflfeggio Tradition)
Yes, but the „octave repeating“ sharps and flats are not much a question of mode, don‘t you think? Sometime in the 18th century they were economized away (is this proper English? - German speaking here )
But maybe I misunderstood your remark?
[Edit: these are two different subjects, I misread your first answer to be related to the multiple f sharps in the same system - sorry!]
See my quote above - I would not go that far to call this „misguided“ if it made sense to them …
… and I’m not sure, why this Inventio would inevitably french style (?)
Not to wanting to appear know-it-all, just curious after decades of reading notation from several centuries (and teaching).
But maybe this is going too off topic …
Just one more for the “wrong” notation of minor key:
an English edition of Corelli (printed 1740), very much not french:
I’ll stop now, promised 
Of course, my answer was full of typically French humour, and you didn’t decipher it… Nevermind. 