Musical note indicated by placing the symbols in the key signature, which then apply implicitly to all occurrences of corresponding notes. Explicitly noted accidentals can be used to override this effect for the remainder of a bar. A special accidental, the natural symbol , is used to indicate an unmodified pitch. Effects of key signature and local accidentals do not accumulate. If the key signature indicates G, a local flat before a G makes it G (not G), though often this type of rare accidental is expressed as a natural, followed by a flat () to make this clear. Likewise (and more
Musical note D, E, F, G, A, B, C and then in reverse order, with no key signature or accidentals. In all technicality, "music" can be composed of notes at any arbitrary physical frequency. Since the physical causes of music are vibrations of mechanical systems, they are often measured in hertz (Hz), with 1 Hz meaning one vibration per second. For historical and other reasons, especially in Western music, only twelve notes of fixed frequencies are used. These fixed frequencies are mathematically related to each other, and are defined around the central note, A. The current "standard pitch" or modern "concert pitch"