Why You Can Never Tune a Piano
Briefly

Pianos are slightly but necessarily out of tune, unlike guitars and violins, which can be perfectly tuned. This discrepancy arises due to mathematical impossibilities intrinsic to the instrument's construction and design.
While string instruments can use equal temperament tuning effectively, pianos must adapt to a compromise known as 'the well-tempered scale', perpetually keeping them quartally out of tune to accommodate different keys.
Read at Open Culture
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