This article is about the drawing and measuring instrument. For the person who rules a country, see Monarch or Lists of incumbents.
A 2 metre carpenter's rule
Retractable flexible rule
A closeup of a steel ruler
A ruler, or rule, is an instrument used in geometry, technical drawing and engineering/building to measure distances and/or to rule straight lines. Strictly speaking, the ruler is essentially a straightedge used to rule lines and the calibrated instrument used for determining measurement is called a 'measure'. However, common usage is that a ruler is a calibrated straightedge that can be used for making measurements.
Types
A Rule has the measurements start from the end, whereas a ruler has a gap between the end and the measurements. Rulers have long been made of wood in a wide range of sizes. Plastics have been used since they were invented; they can be moulded with length markings instead of being scribed. Metal is used for more durable rulers for use in the workshop; sometimes a metal edge is embedded into a wooden desk ruler to preserve the edge when used for straight-line cutting. 12 inches or 30cm in length is useful for a ruler to be kept on a desk to help in drawing. Shorter rulers are convenient for keeping in a pocket. Longer rulers, e.g., 18 inches (45cm) are necessary in some cases. Rigid wooden or plastic yardsticks, 1 yard long and metre sticks, 1 metre long, are also used.
Desk rulers are used for three main purposes: to measure, to aid in drawing straight lines and as a straight guide for cutting and scoring with a blade. Practical rulers have distance markings along their edges.
Measuring instruments similar in function to rulers are made portable by folding (carpenter's folding rule) or retracting into a coil (metal tape measure) when not in use. When extended for use they are straight, like a ruler. The illustrations on this page show a 2 metre carpenter's rule which folds down to a length of 24 centimetres to easily fit in a pocket, and a 5 metre long tape which retracts into a small housing.
A flexible length measuring instrument which is not necessarily straight in use is the tailor's fabric tape measure, a length of tape calibrated in inches and centimetres. It is used to measure around a solid body, e.g., a person's waist measurement, as well as linear measurement, e.g., inside leg. It is rolled up when not in use, taking up little space.
Ruler applications in geometry
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In geometry, a ruler usually refers to an unmarked ruler (a straightedge), only used for drawing straight lines between points, not measuring.
A ruler and compass construction refers to constructions using an unmarked ruler and a compass. It is possible to bisect and angle into 2 equal parts with ruler and compass. It can be proved, though, that it is impossible to divide an angle into 3 equal parts using only a compass and straightedge—the problem of angle trisection. However, should a single mark be allowed on the ruler, the problem becomes solvable.
In heraldry
- The ruler (calibrated, though numbers are not shown) appears as a charge in heraldry, notably in the arms of Odouze.
- The Freemasons are identified by a compass and square device, the latter another type of ruler. Together, the tools represent judgement and discernment.
See also
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