Dihedral Board Aeroplane
A dihedral board on an aeroplane, or airplane, for stability. The boards are raised on short legs over the ribs with a spirit level on top of the board. Dihedral boards are used as a last result due to inaccuracy in stability.
H. Barber The Aeroplane Speaks (New York, NY: Robert M. McBride & Co, 1917)