duplex instruments

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duplex instruments (Brass). These exist in 2 types: (a) those planned to produce 2 qualities of tone by the provision of 2 bells of different bore, and (b) those planned to play in either of 2 different keys (i.e. from either of 2 different fundamental notes) without change of quality of tone (e.g. the double hn. in F and B♭) by the provision of double lengths of valve tubing.