These old rulers were used for navigation and for drafting in architechture as well from what I can find out.  Beautiful timber and brass fittings.
 
Option 1: 47cm L x 8.5cm when closed or 16.5cm fully opened
Option 2: 53cm L x 8.5cm when closed or 16.5cm fully opened
Option 3: 47cm L x 8.5cm when closed or 16.5cm fully opened