selected
The French government passed a law setting the musical note A4 to a frequency of 435 hertz, in the first attempt to standardize concert pitch.
Related Wikipedia articles
A (musical note)
Musical note
A is a musical note equivalent to 440 Hz in typical A440 tuning. It is the sixth note of La and the tenth semitone of the fixed-do solfège.Source: Wikipedia (CC BY-SA)
Hertz
SI unit for frequency
The hertz is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event per second. The hertz is an SI derived unit whose formal expression in terms of SI base units is 1/s or s−1, meaning that one hertz is one per second or…Source: Wikipedia (CC BY-SA)
Concert pitch
Reference point for tuning musical instruments
Concert pitch is the pitch reference to which a group of musical instruments are tuned for a performance. Concert pitch may vary from ensemble to ensemble, and has varied widely over time. The ISO defines international standard pitch as A440, setting 440 Hz as the frequency of…Source: Wikipedia (CC BY-SA)