Ruler

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This article is about the drawing and measuring instrument. Ruler can also refer to a statesman in charge or ceremonial head of state of a country or minor politically significant principality; for this meaning see Monarch or Lists of incumbents.

A variety of rulers

A 2 metre carpenter's rule

Retractable flexible rule

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 the instrument used to rule lines and the calibrated instrument used for determining measurement is called a measure. However, common usage is that a ruler is calibrated so that it can measure.

Several different designs of flexible instruments are used to determine length, such as the carpenter's rule, the ribbon-like tape measure used by tailors, and the retractable rule used especially in the construction trades and by home handyman, also known as a tape measure. As can be seen by the photos on this page, a 2 metre carpenter's rule can be folded down to a length of only 20 centimetres to easily fit in a pocket, and the 5 metre long tape easily retracts to fit within a small-sized housing.

Practical rulers have distance markings along their edges.

The ruler (calibrated, though numbers are not shown) appears as a charge in heraldry, notably in the arms of Odouze.

[edit] Geometric constructions

Main article: Compass and straightedge

In geometry, a ruler usually refers to an unmarked ruler, only used for drawing straight lines between points, not measuring. A ruler and compass construction thus refers to constructions using such unmarked ruler and a compass. It is known for instance, that the angle trisection problem is an impossible one, within the classic restriction of ruler bearing no marks. However, should a single mark be allowed on the ruler, the problem becomes solvable. It is common to call rulers in geometric context as straightedges.