A Dictionary of Music and Musicians/Natural
NATURAL. A word formerly applied to the scale of C major, which was called 'the natural scale' because it has no accidentals. It thus became used for the sign (♮) which cancels a preceding sharp or flat, whether used as a chromatic accidental or occurring in the signature. In other words, when the use of a sharp or flat has indicated that the note a semitone above or below that in the diatonic series of C major is to be taken, the introduction of a Natural indicates that the unaltered note is to be resumed; and hence a naturalised note is always a white key on the pianoforte or organ, unless it be combined with a sharp or flat, as ♮♯ or ♮♭, to cancel a chromatic double-sharp or double-flat, and indicate the corresponding note of the diatonic series indicated by the existing signature.
Naturals do not occur in the signatures of keys, except when it is necessary to cancel all or part of a previous signature, at a change of key in the course of a piece of music; as at the change from C minor to C major in the Marcia Funébre of the Eroica Symphony, or the change from E♭ minor to E♭ major at the end of the Introduction of Spohr's Overture to Jessonda. Where a complete change is made from a sharp key to a flat key, or vice versâ, the naturals are often indicated, but with very little reason, as the mere statement of the new signature must cancel the former one.