The US music industry suggested A440 Hz as the standard pitch in 1925 and it was adopted throughout the US in 1926. In 1955, the International Organization for Standardization adopted A440 Hz as the world tuning standard.

However, it had been suggested and used much earlier than that in isolated areas. The Paris Conservatoire adopted A440 as the "modern concert pitch" as early as 1812.

Pitch is a frequency and is measured in Hertz. A common tuning pitch is A440; 440 refers to the frequency of the pitch.

Alexis says ; The wavelength decreases.

The A440 option on your metronome is to produce the note of A at 440 HZ. It's on the metronome to help you to tune your voice to pitch of the note of A as a starting point.

Specifically A440 is the A in the fourth octave (in orders words A4, the A above middle C).

See the related link for further information.

A440, also known as the A above "middle C", and further known as the note most commonly associated with 440 hertz was made the international standard for pitch decided upon in 1955 (ISO 16), and was reaffirmed by the same organization in 1975.

http://en.wikipedia.org/wiki/Pitch_(music)

I can't find any reasoning as to why A has always traditionally been used for the pitch standard. Even when the tuning fork was originally invented, an A was chosen as the best tuning note. My best guess is that it is due to the note's alphabetic significance.

It should also be noted that although some orchestras tune to an electronic tone that is calibrated to 440 hertz, many do not. Many orchestras are primarily concerned with whether or not they're in tune with each other, and as such, the entire orchestra (or band) often tune to an oboe. The oboe cannot change its tuning, so oftentimes the other members of the band tune to it.

One of the primary reasons for an orchestra, or any group for that matter, to tune to A440 is when there is an instrument whose tuning cannot easily be changed, such as a piano, or organ with a fixed tuning to A440.