Why do humidity and dryness affect wooden stringed instruments?

Humidity and dryness significantly impact wooden stringed instruments because wood is a hygroscopic material, meaning it constantly absorbs or releases moisture from the surrounding atmosphere.1

These environmental fluctuations cause the wood to undergo dimensional changes: humidity causes the wood to expand, while dryness causes it to contract. Because most stringed instruments are constructed from multiple pieces of wood glued together, these changes can be physically destructive.2

Dryness is considered particularly harmful because the resulting contraction can pull the instrument apart, potentially leading to cracks or structural failure. Conversely, while expansion from humidity can also be problematic, it is often viewed as less damaging than the structural stress caused by contraction. To mitigate these issues, makers often use sealers, varnishes, and glues to slow moisture exchange, and they may prefer to build instruments in dry conditions to minimize the risk of future contraction.23