The sound of the 88 piano notes is produced by over 220 strings and the average tension per string is approximately 75-90kg. The combined tension of the strings is approximately 20 tons. Pianos use living breathing materials, that includes pieces of cloth, felt, and buckskin, as well as wool, wood, and metal. Temperature and humidity throughout the seasons change, resulting in changes in the tension and thus changes to the pitch.
*The weather in Hong Kong is a subtropical climate. Typically a piano in Hong Kong should be tuned three times a year.