Definition and Understanding A Notation

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A is an annotation of the extent (6th degree of natural extent of C). Hence Ab, Abb, Anat., A#, A##, A major, A minor, and that. A is annotation commonly used for tuning instrument (orchs. Refrain to the ob. A). A = 440 vibrations per second, globally usual since 1939, although approximately orchestras still acknowledge A = 435 and even in USA A equal to 445.