How long does an accidental affect a note in a measure?

For customary print notation, keep that accidental in effect through the rest of the measure for the same note at the same pitch. The bar line marks the end of that measure; after it, the accidental from the previous measure no longer applies. [S1]

A useful reading habit is to ask two questions whenever the note appears again: “Is it the same note?” and “Is it at the same pitch?” If both answers are yes and you have not crossed a bar line, keep the accidental in mind. This rule is about an accidental printed beside a note; it does not mean every note in the measure is changed.

Here is a text-only example. Assume each letter names a note and the repeated Gs are at the same pitch:

| G♯ A G G | G A G♮ |

In the first measure, the sharp is printed beside the first G. The later Gs in that same measure are also played sharp under the customary rule, even though the sharp is not printed again. At the next bar line, that measure ends. The first G in the second measure is not made sharp by the earlier measure’s accidental. The natural sign printed later in that second measure applies to that G and carries through the remainder of that measure for the same G at the same pitch. [S1]

The example is only a reading illustration, not a claim about a particular song. The vertical bars show the measure boundary, and the natural sign is included to make the reset-and-change distinction visible.

The rule described in the reference is limited to the same note at the same pitch. So do not automatically carry the accidental to a higher or lower instance of that note name; check the pitch as well as the letter name. [S1]

For example, if a sharp is printed beside one G, then a later G at a different pitch is not covered by that rule. Read the notation at that new position and use any accidental printed for it rather than assuming the earlier mark applies. If the pitch or notation is unclear in a particular score, consult its notation guide or a music teacher; this brief rule does not settle every possible notation convention.

Use a small measure-by-measure check. When you meet an accidental, note the affected note and pitch; as you continue, carry it only to matching instances at that same pitch before the next bar line. At the bar line, stop carrying it forward. Then check the next measure on its own. [S1]

You can jot a temporary reminder above or beside your music if that helps, such as G at this pitch: sharp until bar line. Treat it as a reading aid, not as a replacement for the printed notation. The Library of Congress reference cited here is a 1997 Braille music code manual discussing corresponding print notation, and its stated rule is specifically about the same note at the same pitch. It does not establish every special-case convention used by every score or edition. [S1]