Musical term that refers to the pitch A above middle C having a frequency of 440Hz, or cycles per second. This is sometimes referred to as “concert pitch.” Following the principles of equal temperament, A440 means that the C above it has a frequency of approximately 523.3Hz.
A standardized concert pitch is essential for musicians to play instruments together in tune. Some form of standard pitch has been around ever since two individuals wished to sing or play two instruments together. Sometime after 1200 CE, vocal groups (usually monks) and whatever instrumental ensembles existed used a pitch pipe to provide a reference tone for tuning. In 1711, the tuning fork was invented. At that time, it had a pitch of A423.5. This points out that concert pitch has not always equaled A440. In fact, it has varied widely throughout history.
From the 17th century onward, the reference A has ranged from a low of 373.7Hz to a high of 457.6Hz. The differences can be loosely described this way: a lower tuning results in a “darker” sound, while a “brighter” timbre is attributed to a higher tuning. In 1936, the American National Standards Institute (a scientific group, not a musical one) adopted 440Hz. In 1939, A440 was adopted internationally, and it has remained the standard ever since.
Some pitch standards have risen a little, particularly in Eastern European countries, in which pianos are often tuned to A444 or even a bit higher. Some concert halls in the UK and European countries have two pianos on-site, one tuned to A440 and one tuned to A444. This satisfies the desires of performers and keeps the pianos stable.