sixty-fourth note
US /ˈsɪksti fɔːrθ noʊt/
UK /ˈsɪksti fɔːrθ noʊt/

1.
a musical note with a duration of 1/64 of a whole note
:
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The composer used a sixty-fourth note to create a very fast, fleeting sound.
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Playing a series of sixty-fourth notes requires advanced technique.