Aka "concert A," a musical note that is the A above middle C, having a frequency hertz of 440 Hz. One hears it every time an orchestra tunes up at the start of a rehearsal or performance. Traditionally done by the first oboe, as that instrument produces the "purest" tone of that frequency.
http://www.youtube.com/watch?v=gPBgWWZhcA4
http://www.youtube.com/watch?v=gPBgWWZhcA4
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