Transposing piano
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A Transposing piano is a special piano which can be adjusted by the player (e.g. with a lever or pedal) to transpose. There are not many in existence, but they have been used, for example, by people whose skills are restricted to playing in certain keys, or by those who need to transpose music, but lack the necessary skill in so doing.
Some people with absolute pitch may have difficulty playing on such a piano, because the notes they actually hear are different from the notes they are playing on the keyboard. Some such people are able to overcome the difficulty with practice, however.
Irving Berlin had such an instrument.
Many digital pianos and keyboards have wide-ranging facilities, including the ability to transpose.