With wood being the primary material used in their construction, pianos tend to be affected by changes in humidity. Just like the doors in your home that swell up and become more difficult to close, the wood in a piano swells when the moisture in the air around it rises. The most noticeable effect is a change in pitch. As the wood in the soundboard expands, the tension on the strings increases, rendering the notes sharp. Likewise, as the humidity drops and the soundboard shrinks, the pitch becomes flat. Extreme fluctuations can even cause physical damage to the soundboard and other parts, so it becomes critical that the level of humidity around the piano be kept consistent. In an ideal situation, the instrument would live in a room in which the temperature and relative humidity are constant, but since this isn't always practical, another solution is to install a system that will control the environment immediately within the piano itself, maintaining a recommended level of 42%. Such a system can be installed completely out of sight, in both grand and vertical pianos, is easy for the owner to maintain, and will most certainly make for more stable tunings and prolong the life of a valuable instrument.