Accidental (in-bar)
An accidental that applies only within the current bar — unlike the key signature.
What it means
An in-bar accidental (German Versetzungszeichen) is a sharp, a flat, or a natural sign that stands in the middle of a bar before a note and temporarily changes its pitch. It differs from the key signature in two respects: it applies only until the next barline, and it concerns only the notated octave. A sharp before middle F does not act on the F an octave higher within the same bar; that one needs a sign of its own.
In the notation
The sign stands immediately to the left of the notehead, on the same line or in the same space. If the note repeats within the same bar, the sign is not written again — it continues to apply. Notes tied across the barline keep their alteration for the duration of the tie, but the first non-tied note in the new bar again follows the key signature. Where misunderstandings threaten, editors place a courtesy sign in brackets; it changes nothing but serves as a reminder. In sources before the eighteenth century these rules do not apply throughout, and the reach of a sign is there to be clarified case by case.
Musical use
In-bar accidentals are the means for everything that goes beyond the note stock of the key: the raised leading tone in minor, the leading tones of secondary dominants, altered chord tones, chromatic passing notes. In minor, the raised seventh degree as a rule never appears in the key signature but each time within the bar — this is the most common case of all. In terms of function theory, an in-bar accidental changes nothing about the naming of a sonority; it only shows how it is written in the notated text.
The in-bar accidental applies until the end of the bar and only in the octave register in which it stands.
Performance practice
Remember the two limits: the barline and the octave register. Both are regularly overlooked, and both produce persistent errors, because the ear gets used to the wrong note. With ties across the barline, check whether the alteration still applies. And say chromatically dense passages to yourself slowly with note names once, before you finger them.