Jack

The quill or hopper which strikes the strings of a harpsichord or the upright lever in piano action.

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Jagd-stück

A hunting piece.

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Janko keyboard

A system of piano keys in several manuals, or ranks, invented by Paul von Janko. This keyboard necessitates only a small stretch for large intervals.

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Jazz

A significant musical art form which has been developed almost solely in America and is characterized by a great degree of skilful improvisation, distinct rhythmic punctuation and an original approach to instrumentation and orchestration.

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Jeu

Play. Also an organ stop.

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Jig, Gigue, Giga

A lively dance.

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Just intonation

The correct sounding of intervals in singing or playing.