Flat in the next measure, yes or no?

As far as I can say, the cancellation (or conversely the "confirmation" of the original condition of a certain note) of an accidental when it is reapplied in a measure following the one in which it was used, if indeed no accidental is present in the key signature, would not be necessary to specify (but some do, myself included, just to be clearer).

Specifying the same condition even for notes placed at different octave distances becomes necessary in the case where the note in question is altered in the same measure containing "its homonyms" placed an octave or more away, whether in ascending or descending direction.

Then it depends on the context: is the sheet music or score written in a contemporary "language" (for example)?

In this case, there is a tendency to be as precise and detailed as possible, but this is a relative matter and cannot be exhausted in two sentences.

A different discussion arises in the case of having accidentals in the key signature: in this case, the altered notes (for example, by means of a natural sign) remain such until they are returned to their original state (usually by means of the so-called "courtesy accidental," placed between two parentheses).

Sometimes courtesy accidentals (the term "courtesy" already indicates the meaning of such use) are also used about 2/3 measures away from the moment when a change occurred to the state of one or more notes, precisely to "reiterate" the return to the original state.

In classical notation, a transient alteration applies only to the notes within the measure in which it is used; after that, its function ceases. There exists a cautionary natural sign in parentheses to further "clarify" the writing.

In experimental scores from the early 1900s, you find, for example, Berg, who writes an alteration for every note (including the natural sign); today, one refers to the notes, and usually, the notation system used is specified.

Excellent explanations! Everything is clearer to me now.

However, I was thinking about the case of a tie!

If I apply an alteration to a note and tie it for 2 measures, what happens?

For example, a D# held for 2 measures. In the third measure, it returns to being a normal D (I specify the natural as you advised me, excellent!).

How do I specify the D# of the second measure??? With the SHARP symbol or without?? It would be the one tied to the first D#.

Another thing:

In this example, the 3 measures are consecutive, all on the same line in the score.

If the third measure were on a new line!, that is, at the beginning of a new staff line and therefore with the treble and bass clefs and the key signature clearly visible, can I choose not to specify the natural as you say? In short, it is understood that everything starts again without variations.

THANK YOU!

I had forgotten to specify the case of the slur: yes, if you come from, for example, the last quarter of a measure and this remains tied to the first of the next, it is implicit that the note in question remains altered.

At the moment it returns to its original state, place the alteration before it (if you are still within the measure that includes the slur; if instead the change occurs in another measure, it would be advisable to indicate the return to the previous state of the altered note).

In the second case, if obviously the note is tied for these 3 measures and continues to be so at the beginning even in the next measure which contains the key signature accidentals, the reasoning does not change; if the slur ends within this measure ("the one that wraps around"), perhaps from the second quarter onwards (simplifying, but it could also be an eighth, a sixteenth, etc., depending on the meter), let's say you are in F major and the tied note is the Bb present in the key signature, the slur concerns the B-natural; therefore, from the second quarter it returns to being a flat, you must specify it (both if this occurs as in the example just mentioned, and if the B returns to being a flat right on the first quarter of the measure that presents the accidental in the key signature). Since there is then an accidental in the key signature, it is also worth specifying whether the same note at different distances (one or more octaves) loses its alteration or maintains it: if, for example, you want to maintain the Bb an octave above the tied note, you could use a courtesy accidental in parentheses.

I hope that in this case too I have explained it well, since I am still "asleep."

Everything is clear.

So basically, it seems to me that it is always better to be specific.

Theoretically yes, it is better to be clear.

Also because if someone were willing to perform one of your pieces, you should allow them to be able to perform the latter without having any doubt, especially if you cannot be present at the performance/recording.

You're right! Putting yourself in the performer's shoes... or in our shoes when we study a score.

I thank Dante and Piccinesco!

You're welcome, I hope I was helpful and clear.

Returning to write on this topic because I have stumbled upon a case of alteration that I don't know how to handle.

Let me explain immediately:

l'armatura in chiave è in FA#+, ma ad un certo punto della composizione mi trovo per tre battute di fila in tonalità di DO- .

I was wondering if it would be possible to write, for example, E immediately with a FLAT, without having to write E NATURAL FLAT

idem for A.

THANK YOU as always.

But key modulation, if functional, can be changed during the piece. Look at Wolf's Lieder

In fact Tiger, I hadn't understood that "functional" part until a few days ago

Now I am managing much better.

Thank you