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In page Musical keyboard:

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Over the last three hundred years, the octave span distance found on historical keyboard instruments (organs, virginals, clavichords, harpsichords, and pianos) has ranged from as little as 125 mm (4.9 in) to as much as 170 mm (6.7 in).[citation needed] Modern piano keyboards ordinarily have an octave span of 164–165 mm (6.46–6.50 in), resulting in the width of black keys averaging 13.7 mm (0.54 in) and white keys about 23.5 mm (0.93 in) at the base, disregarding space between keys.