Korg DIGITAL PIANO LP-350 Owner's Manual page 24

About stretched tuning
The [PIANO 1/L] and [PIANO 2/M] sounds use stretched tuning. In order to obtain
a more natural sound, stretched tuning tunes the lower notes slightly flatter and the
higher notes slightly sharper than the equal tempered pitches. Acoustic pianos are
normally tuned using stretched tuning.
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