Concert pitch is 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 music history. 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. In the literature this is also called international standard pitch. The term "concert pitch" is also used to distinguish between the "written" (or "nominal"), and "sounding" (or "real") notes of a transposing instrument, i.e. concert pitch may refer to the sounding 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". Modern standard concert pitch 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. History of pitch standards in Western music 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. Pre-19th century 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. Even within one church, the pitch used could vary over time because of the way organs were tuned. Generally, the end of an organ pipe would be hammered inwards to a cone, or flared outwards, to raise or lower the pitch. When the pipe ends became frayed by this constant process they were all trimmed down, thus raising the overall pitch of the organ. From the early 18th century, pitch could also be controlled with the use of tuning forks (invented in 1711), although again there was variation. For example, a tuning fork associated with Handel, dating from 1740, is pitched at A = while a later one from 1780