Concert pitch

Concert pitch refers to the pitch reference to which a group of musical instruments are tuned for a performance. Concert pitch may vary from ensemble to ensemble, and has varied widely over musical history. In the literature this is also called international standard pitch. The most common modern tuning standard uses 440 Hz for A above middle C as a reference note, with other notes being set relative to it.

The term "concert pitch" is also used to distinguish between the "written" (or "nominal"), and "sounding" (or "real") notes of a transposing instrument - concert pitch here refers to the pitch on a non-transposing instrument. Music for transposing instruments is transposed into different keys from that of non-transposing instruments—for example, playing a written C on a B♭ clarinet or trumpet produces a non-transposing instrument's B♭. This pitch is referred to as "concert B♭".

The A above middle C is often set at 440 Hz. Historically, this A has been tuned to a variety of higher and lower pitches.

Historically, various standards have been used to fix the pitch of notes at certain frequencies. Various systems of musical tuning have also been used to determine the relative frequency of notes in a scale.

Until the 19th century, there was no coordinated effort to standardize musical pitch, and the levels across Europe varied widely. Pitches did not just vary from place to place, or over time—pitch levels could vary even within the same city. The pitch used for an English cathedral organ in the 17th century, for example, could be as much as five semitones lower than that used for a domestic keyboard instrument in the same city.

...
Wikipedia