Alidades take various forms but can be as primative as two sights mounted along a pole or string, or at opposite ends of an arc or ring. The sights on a rifle barrel might be considered alidades. Alidades facilitate sighting a straight line, lining up one’s position with a distant object and measuring horizontal and usually vertical angles as well. As a plane table tool, they are attached to scaled parallel rules, placed over a map or chart on a table, typically a table atop a tripod, and used to survey or map a local site. The parallel rule provides the means to draw lines in parallel to the sight direction and to plot features to scale relative to a stationary position. Modern alidades are usually equipped with a telescope and come with or without a vertical arc. Plane alidades with only sighting arms hinged to a ruler were made into twentieth century. Today, the alidade principle of ancient times is incorporated into more sophisticated devises like electronic theodolites and the ‘total station’.