Parallel rulers are a drafting

Technical drawing

Technical drawing, also known as drafting or draughting, is the act and discipline of composing plans that visually communicate how something functions or has to be constructed.Drafting is the language of industry....

 instrument used by navigators to draw parallel lines on charts. The tool consists of two straight edges joined by two arms which allow them to move closer or further away while always remaining parallel to each other.

History

The parallel ruler was invented in about 1584 by Fabrizio Mordente, but it was not in common use until the 18th century.
In the 19th century, a Captain William Andrew Field (c. 1796 - 1871) improved the design by adding a protractor

Protractor

In geometry, a protractor is a circular or semicircular tool for measuring an angle or a circle. The units of measurement utilized are usually degrees.Some protractors are simple half-discs; these have existed since ancient times...

-style scale to the upper edge of one rule, and compass

Compass

A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...

 points to the opposing edge, which made reading bearings easier. Examples exist of boxwood

Buxus

Buxus is a genus of about 70 species in the family Buxaceae. Common names include box or boxwood ....

, ivory

Ivory

Ivory is a term for dentine, which constitutes the bulk of the teeth and tusks of animals, when used as a material for art or manufacturing. Ivory has been important since ancient times for making a range of items, from ivory carvings to false teeth, fans, dominoes, joint tubes, piano keys and...

 or ebony

Ebony

Ebony is a dense black wood, most commonly yielded by several species in the genus Diospyros, but ebony may also refer to other heavy, black woods from unrelated species. Ebony is dense enough to sink in water. Its fine texture, and very smooth finish when polished, make it valuable as an...

, usually with brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 hinges. The instrument usually had two links, but longer models sometimes had three, and sometimes the links were scissored

Scissors

Scissors are hand-operated cutting instruments. They consist of a pair of metal blades pivoted so that the sharpened edges slide against each other when the handles opposite to the pivot are closed. Scissors are used for cutting various thin materials, such as paper, cardboard, metal foil, thin...

. Another variation is the "roller" model which included a cylindrical roller for ease of use.