Tuning and A440

A440 means that the key of A  above Middle C  vibrates 440 times a second  when played.  This is also called Standard , Concert , or Factory  pitch.  There has to be a standard of pitch so that all musical instruments will play notes of like pitch.  It is the same principle that says that every foot ruler must be 12 inches long.  A440 makes it possible for a person with a clarinet made in Germany, for example, to play music while accompanied by a piano made in New York - or any other place.

Tuning of any piano should be twice a year, but
most are tuned only once a year.

Concert instruments are tuned for every performance.
That can mean three tunings for one concert.

The longer a piano remains untuned, the more difficult
it is to tune and stay  in tune.

Tuning