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Why does a piano go out of tune ?

• TEMPERATURE CHANGES. Fluctuations in room temperature surrounding a piano cause less of a change in tuning than humidity changes do. However, direct sunlight or heat from stage lights is so intense  that it can cause rapid changes in the tuning.
• HUMIDITY CHANGES. Although a soundboard has a coating of varnish or lacquer, moisture from the air can seep into and out of the wood, mainly through the end grain, causing the crown to increase and diminish. This is the most important factor that causes a good piano with tight tuning pins to go out of tune. A piano goes flat, particularly in the midrange, in the early winter when the dry heat of the furnace draws moisture out of the soundboard, disminishing the crown. It goes sharp again in the spring when you turn the furnace off for the season and moisture from rain enters the soundboard, increasing the crown. This seasonal pitch change is noticeably absent from a piano kept in a climate controlled (temperature and humidity conttolled) environment.
• SLIPPING TUNING PINS.  This factor doesn`t enter into the tuning of a good quality new piano, in which the pins should be so tight the the string tension doesn`t cause them to turn. In an older piano that has been exposed to regular seasonal humidity changes for many years, however, the pinclock loses its tight grip on the pins. When the pins get looser, strings tension causes them to rotate slowly, over a period of months, allowing the pitch to go flat.
• STRETCHING OF THE STRINGS. New music wire has a lot of elasticity, and it begins to stretch as soon as you pull it up to pitch. New strings stretch the most during their first few years in a piano. Because of this stretching, many new pianos sink a quarter step flat within a few months after each tuning, for the first two or three years. Then the stretching decreases, and the pitch remains stable for longer periods of time. Some piano makers and rebuilders stretch their new strings with a small roller immediately after pulling a piano up to pitch. This precedure helps, but if overdone it is harmful to the strings. The louder and more frequently you play a newly-strung piano, the faster the strings will stretch, and the sooner they will stabilize. ( Arthur A. Reblitz )