Standard pitch
Tuning pitch, today A = 440 Hz.
What it means
The standard pitch (German Kammerton) is the agreed pitch to which instruments are tuned. Usual today is the A above middle C at 440 vibrations per second, fixed as an international agreement. The number is a convention, not a natural measure: it had to be agreed so that instruments of different origin can play together.
Musical use
Historically the tuning pitch fluctuated considerably — from place to place, from church to church, between organ and chamber music. Differences of a semitone and more were normal, and this is not a side matter: an aria that lies comfortably in the tuning of its time of origin can stand at the limit of singability in present-day tuning. Historically oriented ensembles therefore often tune lower, depending on the repertoire to about 415 or 430 vibrations per second; for certain areas tuning is also done higher.
In the nineteenth and twentieth centuries the tuning rose overall, which gives the sound more brilliance but at the same time increases the strain on instruments and voices. Even today some orchestras tune slightly higher than 440.
For people with absolute pitch this has noticeable consequences: music in a deviating tuning appears to them shifted, which can make playing along harder.
The standard pitch is to be distinguished from the tuning system — the former fixes the height of a reference note, the latter the relation of all the notes to one another.
The fixing of the standard pitch has a long history. In the Baroque the tuning pitch fluctuated considerably between cities and instruments, and in the nineteenth century it rose steadily. In 1939, A = 440 Hz was internationally recommended. Today many orchestras tune somewhat higher, at 442 or 443 Hz, while early-music ensembles often use 415 or 430 Hz.
Performance practice
Before every rehearsal, settle to which pitch is tuned, especially when wind instruments or an organ are involved — an organ determines the tuning, because it cannot adapt. Tune only when the instrument has taken on the temperature of the room, and check again after the first period of playing. And, in the ensemble, tune by the sound, not only by the device.