A440, also known as the standard pitch, refers to the musical note A above middle C that is tuned to a frequency of 440 Hertz (Hz). This standard is widely used by musicians and orchestras to ensure that instruments are in tune with one another. The adoption of A440 as a tuning standard began in the early 20th century and has since become the most common reference pitch in Western music. The use of A440 allows for consistency in musical performances, making it easier for musicians to collaborate. While A440 is the most recognized standard, some orchestras and genres may use different tuning frequencies, such as A432, which is believed by some to have a calming effect.