A late eighteenth century English parallel ruler of an uncommonly long length. The brass hinges are baluster shape and the attachment  points are finished with engine-turned rondels.
Length. 456 mm. (18 inches).
Parallel rulers were an intrinsic tool for chartmakers and for plotting navigation points. Most ship's chart rooms would have had at least one and in the case of hydrographical survey ships they would have had a wide selection of various lengths.
Condition. Certainly used but still in tight condition. A slight chip on the end of one of the rules.