Natural sign
The square sign ♮ that cancels an accidental.
What it means
The natural sign takes back a previously valid raising or lowering and restores the basic note. It is thus no accidental of its own in the sense of a change but the cancellation of one: a B lowered by the key signature is made a B again by the natural sign, a raised F an F again. Its reach corresponds to that of an accidental — it holds for the note concerned in its octave position until the end of the bar. (In German the notes here are h for B natural and b for B♭, so a German source’s “natural on b” restores h, English B.)
In the notation
The sign consists of an open square shape with two projecting strokes and stands, like every accidental, immediately to the left of the notehead, on its line or in its space. After a double sharp or a double flat, a single natural sign suffices in today’s practice to restore the basic note; older editions have divergent spellings for this. When the key changes, the natural sign at times also appears in the key signature to indicate accidentals that fall away. As a warning or cautionary sign it occasionally stands in brackets where none would strictly be needed.
Musical use
In practice the natural sign is the key to all chromaticism in the notated texture: without it a brief digression could not be taken back again. In minor it occurs regularly, when the seventh degree raised in the harmonic form is taken back again in the descending melodic form. With secondary dominants and altered chords it ensures that after the excursion the diatonic state is unambiguous again. In chromatically dense passages a single missing natural sign can alter the harmonic sense of a place.
Performance practice
With every natural sign, make sure which note is actually meant — it does not say “higher” or “lower” but only “back to the basic note”, and the direction depends on what held before. Bear in mind that its validity ends with the bar line: in the next bar the key signature holds again, even without a new sign. With fast chromatic runs it helps to speak the place once slowly and aloud with note names before you finger it.