The standardization of one absolute musical PITCH in order to obtain, through consequent TUNING, identical pitches in all instruments for any note held in common by their respective RANGEs.
In Western music, the present day standard of concert pitch is a´ 440, i.e. the FREQUENCY of the pitch A in the first octave above middle C is 440 Hz (adopted, International Standards Association conference, London, 1939). Previously, the standard was a´ 435 (fixed, Paris Academy, 1859, as diapason normal; and confirmed, Vienna conference, 1885, as international pitch). Before that (although hoch Kammerton - chamber pitch, a semitone lower than a´ 440 - was the most common instrumental pitch from ca. 1700 to ca. 1820), different pitch levels were used, particularly for different instrumental ensembles, e.g. choir and organ, the town brass band, etc. A single account of the historical confusion of pitch standards in which every detail can be trusted does not, and may never, exist.
Compare: INTONATION, METRONOME, PERFECT PITCH, TEMPERED TUNING.