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x1 tunes x2 (instrument) to A4 == x3 Hz in temperament x4
«i mi togytikygau le mi cpupi’o li 415 la’oi Vallotti» = I tune my harpsichord to A=415hz in Vallotti temperament. A4 is also called a' (hemholtz pitch notation) and is key 69 in MIDI.