Changes in humidity and temperature cause pianos to go out of tune.

Pianos are complex mechanical instruments containing over 12,000 parts, many of which are made of spruce, maple, or hornbeam wood. The soundboard, typically made of solid spruce, is the most sensitive component to environmental changes. Because wood is hygroscopic, it naturally absorbs and releases moisture from the surrounding atmosphere to reach an equilibrium.When humidity rises, the soundboard fibers expand, increasing the crown or curvature of the board. This upward pressure pushes against the bridge and increases the tension of the steel strings, which can exert up to 20 tons of total force. This physical shift raises the pitch of the notes, often unevenly across the keyboard. Conversely, in dry winter months, the wood loses moisture and shrinks, causing the pitch to go flat.The Piano Technicians Guild (PTG), an organization founded in 1957, notes that even high-quality instruments cannot withstand these fluctuations without maintenance. Fluctuations in temperature also affect the metal strings themselves, though the wood's reaction to humidity is the primary cause of tuning instability. To mitigate these effects, many institutions use Humidity Control Systems, such as the Dampp-Chaser, to maintain a constant relative humidity of approximately 42 percent inside the piano cabinet. Regular tuning is essential because repeated swelling and shrinking can eventually lead to permanent cracks in the soundboard or loose tuning pins.