Piano tuning became a profession around the beginning of the 1800s.
Early piano tuners were trained and employed in piano factories, and often underwent an apprenticeship of about 5–7 years. Early tuners faced challenges related to a large variety of new and changing pianos and non-standardized pitches.
Historically, keyboard instruments were tuned using just intonation, pythagorean tuning and meantone temperament meaning that such instruments could sound “in tune” in one key, or some keys, but would then have more dissonance in other keys.
While unpleasant intervals were avoided, the sizes of intervals were still not consistent between keys, and so each key still had its own distinctive character. During the 1700s this variation led to an increase in the use of equal temperament, in which the frequency ratio between each pair of adjacent notes on the keyboard was made equal, allowing music to be transposed between keys without changing the relationship between notes.
Pianos are generally tuned to an A440 pitch standard that was adopted during the early 1900s in response to widely varying standards.
Previously the pitch standards had gradually risen from about A415 during the late 1700s and early 1800s to A435 during the late 1800s.
Though A440 is generally the standard, some orchestras, particularly in Europe, use a higher pitch standard, such as A444.