A six‐foot grand piano has been studied for the change of its tuning with relative humidity. During nearly two years of observations in the living room in which the piano is located, the relative humidity varied between 20 and 70 percent. The change of tuning lagged the rise and fall of humidity as if the structure were characterized by a “time constant” of the order of 15 days. The relative humidity weighted according to this time constant varied only between 35 and 62 percent. Within the three central octaves, the tuning of this 40‐year old piano rose on the average 5 cents (0.3 percent in frequency) for each increase of 10 percent in weighted relative humidity. A working hypothesis is developed by which to organize the experimental results.