ISO 16:1975, Acoustics, Standard tuning frequency (Standard musical pitch)

ISO 16:1975 is the International Organization for Standardization's own technical standard fixing concert A at 440 hertz, replacing the earlier ISO Recommendation R 16-1955, and its standing rests on that documented act of international technical standardization rather than on any claim about ancient or sacred tuning. Its real evidentiary use here is narrow and checkable: the existence and date of this standard is the fact that actually settles whether a piece of music, medieval chant included, can have been performed at a specific named frequency in hertz before this standard existed. Use it to establish the modern standard tuning frequency and its adoption date; it is not evidence for any claim about a specific historical or mystical significance attributed to 440 hertz or to any other frequency, which needs a separate source addressing that specific claim directly.