NOTATIO

when there is a passage of divisi, for example in a violin section, and a unison note occurs, with an accidental (for example, both parts playing the same C#).
but one part is playing a half note (white notehead) and the other is playing a quarter note (black notehead),

should the accidental be repeated for both noteheads?

or can one single # before both notes be used?

I'm looking at the particular spot in my score, and the extra distance of the 2nd sharp sign pushes the 2nd voice's note far to the right. it looks oddly spaced to me.
But I'd rather avoid any question as to whether or not both parts are actually playing that C as a sharp or not.

Is this for the score only, with two staves in the part? Or is it like this for both score and part? I think I'd just have the one sharp if I were doing it myself, but I can't say with confidence whether or not this is strictly correct (obviously if the part is on two staves they will both need the sharp).

it's from the score, but the part is on one staff, the same disposition.
The divisi is too short to really warrant having an extra staff for that passage.

Never seen a second accidental on a perfect unison.

so the fact that there is no natural sign implies that the quarter note C is also sharp?

thank-you.
this will look much neater in the score/part.

If you're worried, you can add a sharp in parentheses below the lower note's stem.

considering a C natural would REALLY not fit the harmony at that point, I'm going to cross my fingers and hope it's self-explanatory.

a few measures earlier there's an almost identical passage, but with B natural (with no displayed accidental) instead of C#, so it would be natural to assume that the same thing happens in both measures, just on different notes.

(and "oops", the open string indication is no longer needed, it's been removed. the passage was originally done in double stops, written a bit differently.)

MichelRE wrote: ↑28 Aug 2023, 22:45 so the fact that there is no natural sign implies that the quarter note C is also sharp?

To my knowledge, it does and always has.

To be sure, I just checked a B&H miniature score of Tristan and immediately found several of these with one accidental for the unisons, so I think you are safe. But you could certainly add small cautionary below, as suggested by hautbois baryton, to be even safer.

John Ruggero wrote: ↑28 Aug 2023, 21:58 Never seen a second accidental on a perfect unison.

This.

I think this is analogous to the basic rule that an accidental is valid throughout a measure.