Humidity is defined and the composition of air is given. The article distinguishes the various terms dealing with the humidity, such as absolute humidity, relative humidity, dew point, and wet bulb temperature. The meaning of regain and natural moisture content and how they affect the behavior of textiles is also explained. Ways of measuring relative humidity are described. Hygral expansion and shrinkage as functions of natural moisture content according to varying relative humidities is suggested as the cause of bubbling in lined tapestries and rugs.