How long does a sharp or flat last in a measure?

Accidentals last until the bar line, apply only to that line or space, and carry over a tie. What courtesy accidentals mean.
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Band TrainerWritten for band students, by a band director

You see an F♯ at the start of a measure, and two notes later there's another F with no sharp in front of it. Is it still sharp? Yes. A sharp, flat, or natural written in the music (an accidental) keeps working for the rest of the measure. Here are the rules, and the few cases that trip people up.

The bar line rule

An accidental lasts until the next bar line. Every later note on the same line or space in that measure gets the same sharp, flat, or natural, even though it isn't written again. When the bar line comes, the accidental is cancelled and the note goes back to whatever the key signature says.

F♯GF♯still sharpAFnew measure
The third note has no sharp written, but it's still F♯. After the bar line, F is back to F.

Same line or space only

An accidental only applies to notes on the same line or space. A sharp on the F in the bottom space doesn't change the F on the top line, even in the same measure. If the composer wants both to be sharp, both will have a sharp.

F♯Fnot sharp
Same letter, different octave: the top-line F is a plain F.

Ties across the bar line

There's one exception to the bar line rule. If a note with an accidental is tied across the bar line, the tied note keeps the accidental. It's still the same note, just held longer. But the next new note on that line or space goes back to normal.

F♯F♯heldFnew note
The tied note is still F♯. The F after it is a new note in a new measure, so it's F natural.

Not sure whether a curved line is a tie? See ties vs. slurs.

Naturals and cancelling

A natural sign (♮) cancels a sharp or flat, whether it came from the key signature or from an earlier accidental. It follows the same rule as any accidental: it lasts until the bar line.

B♭from keyB♮B♮still naturalB♭from key
With one flat in the key signature, every B is B♭. The natural makes it B for the rest of that measure only.

This is where most missed notes come from in band: the key signature says one thing, an accidental changes it for a measure, and it's easy to keep playing the accidental after the bar line. For more on key signatures, see key signatures.

Courtesy accidentals

Sometimes you'll see an accidental that isn't technically needed, often in parentheses. That's a courtesy accidental (also called a cautionary accidental). It's a reminder, usually right after a bar line, that an earlier accidental has worn off.

F♯()Freminder
The natural in parentheses doesn't change anything. F would be natural anyway; the composer is just reminding you.

Courtesy accidentals don't change the rules. If one is missing, the bar line rule still applies. Many players pencil in their own when they keep missing the same note.

Practice

  • Before you play a new piece, scan each line for accidentals and circle any note later in the measure that they affect.
  • At each bar line after an accidental, ask: does the next note go back to the key signature?
  • Pencil a courtesy accidental over any note you miss twice.
  • Watch for ties: a tied note keeps its accidental across the bar line.

The faster you can name notes, the more attention you have left for accidentals. Build that speed in Note Trainer.

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