<P> The word in Arabic (الحلقة العضدية al - ʿđ̣udiyyah, "the ruler"), signifies the same device . In Greek and Latin, it is respectively called διοπτρα, "dioptra", and linea fiduciae, "fiducial line". </P> <P> The earliest alidades consisted of a bar, rod or similar component with a vane on each end . Each vane (also called a pinnule or pinule) has a hole, slot or other indicator through which one can view a distant object . There may also be a pointer or pointers on the alidade to indicate a position on a scale . Alidades have been made of wood, ivory, brass and other materials . </P> <P> The figure on the left displays drawings that attempt to show the general forms of various alidades that can be found on many antique instruments . Real alidades of these types could be much more decorative, revealing the maker's artistic talents as well as his technical skills . In the terminology of the time, the edge of an alidade at which one reads a scale or draws a line is called a fiducial edge . </P> <P> Alidade B in the diagram shows a straight, flat bar with a vane at either end . No pointers are used . The vanes are not centred on the bar but offset so that the sight line coincides with the edge of the bar . </P>

Working edge of an alidate is known as