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counter-diapason

ˈcounter-diaˌpason Mus.
  [counter- 12.]
  An organ-stop an octave lower in pitch than the ordinary diapason.

1852 Seidel Organ 94 Large organs have sometimes, in the great organ, both a diapason eight feet and one sixteen feet, the latter being then called double-diapason, or counter-diapason.

Oxford English Dictionary

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