A ruler is an instrument that can be used for measuring distances or to draw straight lines in printing, geometry, technical drawing and many other things.
Throughout the history, humans usually take their parts of the body as measuring units. Such units are hand, foot or cubit. The metric system was invented in France for easier measuring and by the end of 18th century most of the countries the world adopted it. People are using things to help them measure something for more than four thousand years.
First ruler war a measuring rod made of copper alloy and it dated from 2650 BC. That measuring rod was found by German assyriologist Eckhard Unger. In 1500 BC there were ivory rulers used by the Indus Valley Civilization. Some findings in Lothal proved that there was a ruler which was calibrated to 1.6 millimeters, and it was 4400 years old. Mohenjo-Daro ruler has accuracy within 0.13 millimeters. The first folding ruler was invented by Anton Ullrich, and the first flexible ruler was made in 1902.
Rulers come in a different shapes sizes and materials. Some of that materials are plastic, wood and metal. Metal, as a stronger material, if often used for a workshop rulers. Rulers that are used for office work are usually made of wood. Rulers can be made in different sizes. Usual size for a desk ruler is 30 cm. Longer ruler size is 45 cm. There is also small rulers that can fit in a pocket and carpenter rulers that can be folded in a zig-zag shape. For a larger measuring distances yardsticks (1 yard) and meter sticks (1 meter) are in use but today it is more and more replaced by tape measure and laser rangefinders.
Three main usages of desk rulers are to measure, to draw lines and as a straight guide for cutting with a sharp blade.
A line gauge ruler is a ruler that is mostly used in the printing industry. Units on that kind of the ruler are millimeters, inches, agate, picas, and points.
One of the specialized rulers is a shrinkage ruler. That type of ruler is designed for making a wooden mold for a metal slightly bigger that the finished product because of the shrinkage rate of the metal.
Rulers don't always measure straight line. Sometimes, like in tailoring industry, it is needed to measure a person waist measurements for example, so there is a folding measurement instrument for that purpose.
There is also a virtual ruler, a ruler software or a screen rulers. It can be used to measure pixels on computers or mobile phones. Contrary to that, there is also a ruler for drawing curved lines. That kind of ruler is known as a French curve. There is also a flexible device known as a flat spline or flexible curve which can be bent into a desirable shape.